The game doesn’t adequately signal that sinking in lava while fire resistant will become fatal given time. There are a few possible solutions depending on how much the model of “dissolved in molten lava” is changed:

• Messages involving sinking in lava should indicate more clearly to the player that even though they are fire resistant, they will eventually die by sinking below the surface and dissolving. Possibly separate messages when the timeout is getting low, such as “You are beginning to dissolve.” and “You are dissolving faster.”
• Sinking in lava while fire resistant is fine, and doesn’t kill the hero if they have magical breathing. The death that occurs if they don’t is changed to “drowned in molten lava”. This could mean the hero can swim through lava so long as they retain both fire resistance and magical breathing.
• Sinking in lava dissolves parts of the hero’s body as they go deeper, causing progressively increasing amounts of HP damage, probably fractions of maximum HP, which eventually go over 100% as they slip fully underneath and kill them outright even if they’ve been healing the damage every time. Messages would also need to be updated.

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.