Call of Duty: Black Ops 4

There are two locations where you can swim underwater in this map. For both you will need to grap the artifact at the far end of the ship first (it cant be missed).
Your first underwater location is right under the artifact. It is the engine room.
The other location is under the starting area at the very front of the ship. It is called the Cargo Room.Both areas are locked behind pretty expensive doors (around 1750 points on solo). Naturally you should not drain the water.

I would recomment going to the Cargo room. You can swim around with plenty of space and can pretty much go up anywhere for air.

The rest should be self explanatory. Grab a good gun, swim underwater and kill every zombie you see. It should only take about two or three rounds.

You can do this on casual difficulty. I would recommend not taking any bots for this as they tend to steal kills.

Use the first 4 rounds to knife zombies and save as much money as possible. Avoid nukes. I like to shoot them before I knife them for extra points with the welling. You should get to the poop deck and grab the Sentinel Artifact which you will need to open the cargo hold. Once you grab the artifact head back to the front of the boat. At this point you should be saving money to grab a weapon you want and open the last door to the cargo hold. The VAPR in the mail room and the Titan on the back cargo hold wall should work fine.

This can be done in casual.