Iron golems rust in multiple stages, like iron items, rather than all at once. Each stage cuts its maximum HP by 20 (so a rusty golem has 60 max HP, etc.) Applying a can of grease will remove the rust and restore its max HP to normal, and may prevent further water damage for a little while. The golem may also quaff a potion of oil, which has a similar effect, except it also restores current HP as well as maximum by the amount of erosion removed.

Iron golems that rust to death may rarely leave behind a statue (ideally made of iron material) which is implied to be its rusted husk.

Magic markers are susceptible to water damage, getting their charges reduced when they get wet. This could be either a few charges, or could possibly go up to the level of halving the remaining charges.

An artifact amulet of magical breathing which also completely protects your inventory from water damage when worn.

Scrolls can be “corrupted” or “misspelled”. This basically acts as a toggle on whether the confused behavior of the scroll activates: reading a corrupted scroll while not confused will produce the confused effect and reading it while confused will produce the regular effect. There is no way to restore a corrupted scroll to normal.

Possibly, an unidentified corrupted scroll will have its label deterministically garbled similar to engravings, e.g. “scroll labeled FO0B|E BLe7Ch”. If identified it would simply show as “corrupted scroll of X”.

Writing a scroll while confused will make the result corrupted. Also, scrolls hit by water damage or produced from polymorphing other scrolls may be corrupted.

When a giant deals you a crushing blow and knocks you back, it can propel you into water you are flying or levitating over, wetting your inventory.

Deliberately dipping a bag into water wets all of its contents with no random chance of failing.

Oil lamps hit by water damage lose some of their oil on account of the water flushing some of it out, reducing the amount of turns of light they have left.

Inventory wetting triggers every turn you are submerged.

Rename rust traps to “water traps”, and the gush of water hitting you may hit your pack, wetting several items.

When dipping an oilskin sack in water, give a unique message “The water runs off the [bag/oilskin sack], leaving it dry.”, which unambiguously identifies the oilskin sack.

Change how drowning attacks work, by making them pull you into a water square that is adjacent to both you and the attacker, and then sticking to you so you can’t move out of the water. This requires that the hero be allowed to survive (with a suffocation timer) underwater for several turns without magical breathing. If there are no mutually adjacent water squares, the attacker can’t use their drowning attack (it would either do nothing or you’d still be held but not get pulled anywhere). Unlike regular drowning, this wets your items each turn.

The hood on a worn dwarvish cloak will protect your helm from getting rusted.

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.

Water elementals deal active water damage to the player, more broadly than rust monsters: blanks scrolls, dilutes potions, extinguishes candles, etc.

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