Stranded Deep

The other solution is great but I have a better way to fight the bosses. It's also worth mentioning the higher your hunting skill the more deadly your attacks will be. I did all bosses with a hunting skill of 6. Also be at full health with a full water and food meter...at least when you depart for the boss. You may get hungry or thirsty by the time you get back but you shouldnt die as long as you go to a boss and straight back, or take food and water with you but I didnt. The spear gun base dmg is 20 along with the crude spear and the refined spear is 40. The number on the spear is THROWN damage not jab damage which is lower by a value of 10 I think.

For the meg you can use your personal crafted raft and come home in it. If you pit your raft between the mast and the ship on the side of the ship that's going WITH the tide your raft will stick there for the fight. So circle the meg and kill all sails before engaging and determine which way youre floating at sea. You want to stick your raft into the wreck so the tide cant push your raft away. Youll want to stand in the very corner of your raft that is pitted against the ship, right near the mast (so on your raft but as close as you can possibly be to the corner that the mast and ship form) the meg cant jump out at you here. At least it never did to me. He couldnt get the angle on me and spent the entire fight under my raft and circling me. I just had full bandages and the speargun in my quick slots and one inventory slot with 24 speargun ammo and the rest full of crude spears. I never had to use the gun. The meg never hit me.

For both the eel and lusca I put my life raft on my crafted raft and sailed to an island closest to the bosses. That way I was only stuck paddling for 1 map square. Anchor anywhere close the the boss fight and dive down to attack and start the fight.

On the eel I had bandages and speargun in the quick slots again and 3 inventory slots full of speargun ammo. No crude spears. When you agro the eel and he comes out IMMEDIATELY turn around and head back to the ship cabin you can climb on and STAY in the water and except for coming up for air stay underwater. This cabin is your savior. The eel will circle the ship in one direction until you damage it. When its hit, it will turn 180 degrees and turn around. You do the same. So this is basically cat and mouse. You chase the eel and keep his tail in your sight. You ARE faster than the eel is. When you have a shot, hit him and turn around and start sprint swimming the other direction all while hugging the cabin near the surface. When you need air get it but KEEP circling on the surface. Go back down after you get air and continue chasing. The eel never hit me and when done I had like 21 or so speargun ammo left and I even missed a few shots.

For Lusca, I used custom island farming for refined spears and had 7 full inventory slots full of them along with 1 slot of speargun ammo just in case. Quick slot the spear gun and bandages like normal. Sail to the closest island to the bouy and save before life boating to the bouy. Anchor close and again, stay in the water just below the surface and only come up for air while keeping the bouy between you and lusca.

Edit: I watched a video of someone cheesing lusca and they pretty much kept their back right up to the bouy and only moved the right stick to aim. Sat completely still and lusca never hit him. So the bout definitely acts as a shield in this fight.

The idea is the same as the eel however...lusca does what he wants and goes in different directions. Sometimes he'll be chasing you, sometimes you'll be chasing him. Hit him with those dummy thick refined spears when you have a shot. He will change direction constantly but as long as your right against the bouy you should be fine. This isn't fool proof however as lusca did hit me a total of 3 times and drug me away from the bouy. If this happens, remember that during this drag away animation you can jab him with the spear. It was cheek for cheek which feels better than him getting a free hit on you. If he hits you bandage up before throwing another spear. Lusca will hurt you and you will die pretty quick. I never had to use the speargun, missed once or twice with spears and still had some left over.

All bosses are pretty risky fights on your raft or on top of their fight areas. Use your environment to your advantage and come prepared and you'll win.

Call me Ahab

Unfortunately with this achievement I can only give you tips and a bit of guidance. The location of every sea monster is different no matter what seed number you have. Consult your cartographer map on the main menu to find their locations. Get to know your map well, (Landmarks like ships, odd shaped islands so you can get your bearings and find out where North, East, South and West are. A good tip is to take a photo or jot it down on some paper.

The requirement is to kill the 3 following sea monsters-

Megaladon (Shark)
The Great Abaia (Eel)
Lusca the Great (Squid)

Keep your yellow dingy with you for the whole game, it is almost a necessity for the bosses. Keep it with you on your main base. I personally had my main base right next to the 'A Way Out' island (This is an aircraft carrier) for bearing reasons. I personally made my own raft that I used for pretty much everything except for the boss fights. If you use the dingy for a boss fight you can anchor in the middle of the ocean without it drifting away but unfortunately with a self made raft can't anchor in the middle of the ocean meaning you'll almost certainly lose it during the battle.

The technique for all 3 are largely the same. Crafting crude and refined spears and throwing them at the monsters. The crude spears only require one stick to crafted. The refined spears require, one crude spear, one leather and one stone tool. Leather is very hard to come by so I found the following method will help farm leather (And any other materials).

Pick any island that you want to farm on. Back out to the main menu > Cartographer > New Island > Animals Tab > Toggle to the wild boar setting to 2/2. Apply the settings, back out and save, chose any name and any description and once done go to World Editor, and 'Replace Island' on the island you want to replace, back out and save. What you'll find is that custom island is now where you put it on your map. GO to the island, collect the materials you need, go back to your base, save and quit. Go back to 'World Editor' click on the farming island once again and click replace island > edit island, once again, on the animals tab press apply and save the map. You'll find the boars have respawned along with any other materials you use. As a tip, you can do this for any farming whether it be ships to look for loot, clay, rocks etc.

The sea monsters are in the middle of nowhere, there is no island but one you get close to their area something will spawn. Hook up your sea anchor on your yellow dingy before the fight and initiate the boss fights by either getting close to them or attacking them. Make sure you're accurate with your shots, especially on the squid. Keep a track of the monsters in the fight at all times. *Quick tip, take a sleeping bag with you just before the boss fight and save, it'll save tonnes of time incase you die*.

Crude Spear damage inflicted - 20HP
Refined Spear damage inflicted - 40HP

First up, the Megaladon - HP 500 - Found by a wreckage and a whale.

For this you don't actually need refined spears, I personally used an inventory full of crude spears, I also took 4 bandages in the last slot incase of bleeding. The trick with this shark is to stay close to the surface as possible. Right under the wreckage it seemed there was a sweet spot where it couldn't drag you away. Just keep an eye on your air though. I wouldn't try to stay out of the water as the shark with jump at you and is pretty effective. It will take 25 crude spears to kill. It may take a couple of tries but with a little persistence it's not too hard.

The Great Abaia - HP 750 - Found by a Boat underwater.

To start this one, drive to the bottom of the ocean and a health bar with come up indicating the fight has started. Immediately go to the stern of the ship. Essentially what you want to do is hide round corners and throw spears when you have a good enough aim, use the boat as cover. The Eel is quick, it's attacks just drag you away and can cause bleeding, try to get back to the stern as quick as possible to carry on hiding and throwing. Again, with a little persistence it's not too difficult. For this boss I had 12 refined spears, 4 bandages and the rest crude spears.

Lusca the Great - HP 1000 - Found chilling on an anchor.

First, you'll want to anchor your yellow dingy directly next to the metal structure. You'll find the squid down on the bottom of the anchor, be cheeky and get a couple of spears thrown while it's down there. As soon as you've hurt it get straight out of the water, you really want to get out fast as the squid is super deadly. The idea is the get onto the metal structure at the top by getting on your dingy first and jumping over to it. You'll want to throw spears directly at it when you're on the metal structure, as it skims across the top, that's your opportunity . As soon as you've damaged it, move either left or right as one of its tentacles will try to knock you off back into the water, if you do fall in, get straight back on your dingy and back onto the metal structure as quick as possible. It's just a case of spear thrown, dodge, spear thrown, dodge. This will probably take a few goes but isn't too bad if you've saved before the fight. For this I used a full inventory of refined spears and four bandages, after the fight I was left with only two spears so make every one count.

Once you've killed each boss you'll unlock two things, an aircraft part and a trophy. Craft all three trophies to receive your achievement.