Minecraft

Very easy.

First, find a kelp plant on the ocean and break them to harvest the kelp leaves (they rise to the surface of the water)

Second, put the kelp in a furnace and cook them.

Now, just eat them one time each day, during 3 days on game (20 min each day so 1 hour)

The achievement pop on the morning of the third day.

ALL in one Guide
Been getting quite some request for an achievement world. I hadn't made one until now due to me not having a lot of free time to dedicate to this as well as it not being as easy to set one up for this. I've tried my best to make a world for this, by using the seed provided by fellow TA member Dwaggienite. It will take more effort to get the achievements in comparison to earlier updates, so don't expect it to be as easy as that. This has to do with the requirements of 3 achievements not being able to set them up as easy.

Also, take note that this world was created in version 1.5.04 and therefore does not work in versions below that, like 1.4 (which for example WP, Kindle, Ios and Gear VR run on as of now, until 1.5.04 or a higher version is released to the public. ) The world can be downloaded right here:
https://www.dropbox.com/sh/90bviwj4dhz8i83/AAATWKpkwXEVNQ8ai...

For windows 10 and windows phone 10 use:
W10_Aquatic Sh0wt1m3 v2.mcworld

For all MCPE versions on phone/tablet use:
MCPE_ Aquatic Sh0wt1m3 v2

Note: There is a bed which you can sleep in, which will change the spawn point for the host.
Any player joining in will have to walk for 2 minutes or so anyway to reach the cabin.
Note 2: might be worth swapping to Experimental gameplay on if achievements fail to pop. Regardless of which gameplay you use, they are buggy and can take a few tries or minutes to pop.
Note 3: Cabin coordinates are 200, 65, -320
Note 4: Unsure how to import worlds, Android has slightly changed but the rest of the ways to import are mentioned in the walkthroughs' hints and tips pages.

Now on to the achievements:
Be sure to load the world on HARD difficulty. Then jump/run around for a bit until you can eat a dried kelp leave. Then swap back to peaceful. This will start the timer for the Castaway achievement. Don't close the game or die for the next hour and this achievement is yours.

Enter the Achievement cabin up ahead via the ladders. Immediately on our left grab the kelp block from the chest and power the furnace with it. Then grab the dried kelp leaves as they are dried for Alernative fuel.

Head down towards the chest on your right. What worked best for me to pop Me gold was first take the treasure map in my hand, then open the chest while having the map in my hand. Then take out a few items, close the chest and open it again to remove the remainder of the items. Then close the chest and wield the iron helm/chestplate/legs/boots.
Note: Reload world if multiple players/platforms need it. Also if it doesn't pop reload a fresh world as once the chest is opened and the achievement popped or not, no one will be able to pop it afterwards.

Now for I'm a Marine Biologist, there is a little pond in the back. Take an empty bucket in your hand and use it on one of the fishes which are in there. (right mouse or tap and hold finger on touchscreen)

For sleep with the fishes, there is a little workaround for that on peaceful difficulty. Sometimes when you go underwater and the oxygen bubbles have all gone down, you only lose one heart which regenerates before you lose a new heart. Keep going underwater until this situation appears, then just stay under water for 20 minutes to pop this achievement.

Now for Atlantis and Ahoy!, there is a shipwreck at coordinate 539, 63, 31 and a ruin at
611, 47, 52. 2nd number is height, so focus on the first and third number to get to these spots. Atlantis always popped fairly soon for me, just swimming past worked. For Ahoy!, I would always sit on the mast in the back of the ship (longe pole) until the achievement popped.
Be sure to keep an eye on your hearts that you do not drown, which will reset the progress for castaway. As a side note, I'd like to point out that Ahoy! would only pop for me on certain version while experimental gameplay was turned ON.

Now you should just have Sail the 7 Seas left. (assuming castaway has popped, if not idle the remainder of the hour for castaway or if you're confident enough not to die just go for the 7 seas.) In this bay/ocean with the shipwreck and ruin in it, swim around for a bit. I've had the achievement pop twice for me at around 570, 50, -70, after swimming through this area. Don't swim in straight lines, but rather 't swim in straight lines but rather meander, thereby also touching the ocean floor every now and then. This should be around 2-3 of the ocean biomes done. For the remainder, we are going to go to these coordinates provided by Sashamorning:
Coordinate 1 = 192, 63, -303
Coordinate 2 = 125, 62, -192
Coordinate 3 = -40, 63, 22.

He's provided a few extra ones but I've only had to use these extra coordinates twice, though the achievement never popped there for me. The route I would take after exploring the shipwreck/ruin area, is first to the cabin: 200, 65, -320. From there, make your way to coordinate 1, then 2, then 3. While doing so, again meander over the ocean floor. Be sure to move to different types of ocean floor as you pass by them ( sand, gravel, stone, dirt), different types of vegetation, (long kelp plants, small grass type of plants, no vegetation), different altitudes (deep ocean and ocean floor relatively close to surface) as well as areas with different types of fish (dolphins, salmon, small fish, no fish, etc.). It's all about having the game registering that you have been in all areas. It is very well possible that the achievement does no pop once you have swum around coordinate 3. Then I would suggest backtracking to the other coordinates while exploring stuff on your way.

Hope this was helpful, feedback is welcome as always.
Credits to Sashamorning, Dwaggienite and JumboWario