Will the campaign be fully aquatic or semi-aquatic? That is, do you also need to be able to breathe air, and is a poor land speed an issue?
You can check
this list. I list subtypes, and anything with the aquatic subtype can breathe water. If a creature has the amphibious special quality, allowing it to also breathe air, I've noted that as well. If a creature has water breathing or breathless as a special quality, I've noted that too.
The amphibious and aquatic templates applied to normal races also merit consideration.
Note also that warforged and necropolitans don't need to breathe by virtue of type.