A spell that grants temporary magical breathing. This could be used in lieu of the magical breathing amulet to go underwater for short durations, but would also be very useful in variants that have suffocating engulfers.

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

Gillyweed: comestible that grants temporary intrinsic magical breathing when eaten.

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.

Fish scale mail; awful AC but grants magical breathing.

A diving helmet item which confers magical breathing when worn.

Artifact chaotic amulet Amulet of Splendor: provides every single amulet effect, as well as unchanging-ignoring controlled polyself when invoked. Wearing it conveys life saving (this WILL disintegrate the amulet if triggered), magical breathing, unchanging, poison resistance, ESP, reflection, strangulation (doesn’t kill you due to magical breathing but maybe it makes it so you can’t eat food or something while wearing it, or it autocurses), restful sleep, and gender change for as long as you wear the amulet. Base type is a cheap plastic imitation of the Amulet of Yendor.