Puffer fish, a monster with a non-poisonous bite but a passive poison attack. Its corpse conveys poison resistance pretty reliably but is always poisonous to eat, even if it’s been tinned.

Eating any kind of fish may increase your Intelligence, because of the popular wisdom that fish is “brain food”.

Confused sea monsters should sometimes randomly move out of the water and flop onto land. (E.g. it suppresses the behavior of treating non-water spaces as invalid.)

Add whales, which can engulf you. Several of them spawn on the Plane of Water.

If the player shoots or zaps something and there is a submerged or hiding monster the player doesn’t know is there in the way, it just flies past without interacting.

New artifact Fishing Pole, some sort of polearm that deals double damage to all sea monsters and can hit squares as if you were Expert (in the full 2-square-wide ring around you), regardless of your polearm skill.

Piranhas and other sea monsters’ bite attacks can only hit you if you’re actually in the water, in a water space next to them.

Goldfish, a ; monster that is level 0, always generates peaceful and has no attacks. The purpose of having it is for flavoring special levels, especially quests.

New artifact: The Trident of Poseidon. Chaotic (only because Poseidon is in-game). While carried, grants magical breathing, swimming, and protects items from water damage. When wielded, confers water walking. Double damage versus aquatic monsters, and gets a damage bonus versus all that scales with the amount of water squares adjacent to you. Invoke to get an UnNetHack-style flood.

When you are polymorphed into a sea monster, you can sit on fountains and drink potions of water to regenerate HP.