I have an odd idea for a setting and I would like help. Why would the safest place to take sanctuary from some sort of monster apocalypse be underwater? I can not get this idea out of my head. In a world lost to zombies, or some other sort of monster(s), a group of survivors find that the best place to spend the night is underwater in a sub. The only thing that I can currently think of is that there is a fog at night that is the cause of the threat. I am not yet sure if the pre apocalyptic setting is modern or closer to cyberpunk. At the very least making me answer questions would help me create the setting. Thank you in advance.

The first thought that comes to my mind is a (non-sentient) nano-swarm that roams the literal surface of the world that infects and transforms those it encounters.

Its programming causes it to traverse 5 feet (1.5m) above the surface of both water and land (why is that? Maybe the PCs can find out!) and its trajectory is random, so you can't predict where it will strike next. Being underwater (or underground, but if there is a way down, the swarm will traverse the underground caverns as normal since it's still the 'surface' of the earth).

Thus, underwater is the safest place to be.

Second thought was a less high-tech option: the apocalypse mutated animals and insects to be carnivorous. With everything on the face of the earth out to eat you, being underwater is the safest place to be. This also works for zombies, since corpses don't sink very well. Again, the trick is something that can't be predicted and has limited/no access to what lies beneath the waves.

(Dis)Honourable mention: it's the aliens from Signs ! They are allergic to water and thus don't traverse the seas nor delve beneath them!

The Polaris game (a French RPG) has mankind return to the seas to avoid extinction. Humanity lives in underwater cities and nations to escape from mutagens, toxins, and radiation. Under the seas, there's politics, scheming, and all sorts of conflict. There's also a sterility issue threatening humanity to add some bleakness. Think of an underseas Firefly with a sub instead of a starship along with power armor for marines, genetic engineering for some wierd creatures.

The rules are a mess, but I was considering using Genesys for it, so...

Start here: https://www.drivethrurpg.com/product/178184/POLARIS-RPG--Quickstart-Guide--Adventure