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.

The water walking property protects inventory from water and protects against drowning attacks.