There are adverse effects if you swim within a certain amount of time after your last meal, because of the traditional advice that it’s dangerous to swim less than an hour after eating.

Artifact cloak Mantle of the Manta Ray, inspired by a similar D&D cloak. It confers magical breathing and swimming when worn.

Wearing sea dragon scale mail grants extrinsic swimming.

If you have swimming, you can use , to pick items up from underwater.

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.

Swimming skill: how well you can swim on top of water and carry weight at the same time. At Unskilled you can carry very little, but it quickly increases beyond that.

Swimming while satiated can cause you to drown from stomach cramps.

Give some monsters the M2_SWIM ability: rats, possibly dogs, possibly horses.

Tourists should be able to swim. Alternatively, tourists should specifically NOT be able to swim, but other roles might.