Putting in on/in a boat or near the coast might get you the seawater for cooling, but the project lists a few other big benefits like oxygen-free atmosphere, constant temperatures year-round, and ease of deployment.
A hermetically sealed capsule can be placed on a boat just as easily as a submarine, if not easier due to the pressure difference.
I'll give you the constant temperature but this project is about using the nearly-free ocean to maintain temperature so surely the delta isn't too large here.
I don't see how this is easier than a boat at anchor.