Minecraft wiki entry for Best_biomes_for_homes, associated text: Unless the player explicitly wants to live underwater, building on a nearby beach may be a better idea if the player is having trouble dealing with air loss or lack of resources.
While drowned are the only outright hostile mobs, pufferfish can inflict poison damage if you get too close, and dolphins will become hostile if attacked like wolves.
Build a conduit will solve all the problems of underwater living. A beacon will massively boost the player.