Water has some quirky behavior associated to it.
(1) Paper and iron items dropped in rivers' water squares do not drench or rust, yet they do in the underwater cave. (2) Fluff balls thrown at or dropped in rivers do not explode into a bunch of gremlins. (3) There is a bug in the underwater cave that causes webs to disappear "due to great heat".
(4) Freezing the water tiles at the bottom of the old barbarian's glade also makes the water in the "nothing special" squares freeze, even though I didn't use Frost Bolt on those.
(5) Fire creatures, notably fire elementals and fire vortices, can wander the WDL just fine without taking damage. (6) Torches of both kinds can be lit in the WDL and will give fire light(!). Everburning ones could be excused by their magical nature, but this shouldn't happen with regular torches.
(7) Monsters stuck in and surrounded by water (because they were in an ice bridge but the PC melted it with a fire bolt) don't suffocate, they just stay afloat forever. (8) You can't wish for monsters if surrounded by ice bridges only. It just gives the "for some seconds a ghostly figure..." message thrice.
(9) Water breath attacks from monsters kill gremlins (which is okay, see #14 below) but they don't multiply. (10) There is no weight check when you teleport to an ice bridge. It's not checked if you use 5 or w5 to wait one turn or in place either. Only when the player moves from another spot to the ice is the weight checked. Similarly, ice bridges can't hold a PC carrying more than ~2000s of items, but multiple suits of moloch armor lying on the ice won't break it. Auto-picking up those moloch armors when the player steps in the ice doesn't break it either. (11) * removed, see http://www.adom.de/forums/project.php?issueid=751 * (12) Ice breath attacks from white dragons don't freeze water, unlike exploding ice vortices or frost bolts from ogre magi.
(13) (Possibly not a bug) Fluff balls can turn into gremlins when crossing a water square in the wilderness. This has no consequences, short of the message displayed and losing the fluff balls. (14) * removed, Thomas has stated in the old bug database that water monster are still affected by the kinetic energy of water breath attacks * (15) Monsters that "ignore water" never drown in the WDL. So rats, giant bats and the cute dog can breathe water there.
(16) Monsters in the WDL can be colored blue sometimes. This is a bit random and may depend on where they're standing. (17) You can't pick up items outside of Blup's mom's LOS, but they can still be kicked, something you cannot do in her presence.
(18) Holes can be dug underwater, and ^ will be shown instead of = in their tiles if the PC does so. The (so far not reported) covered pit bug can be abused normally, and will display a caret of corresponding color afterwards. Water remains there, and is still mentioned when the PC attempts to move to its tile, swims on or 'l'ooks at it.
(19) Fluff balls don't explode if you drop them, then trigger a water trap in the same tile. (20) Visible blood (NotEye) should vanish faster if monsters are killed on water squares.
How to reproduce it
It'd seem to me that the WDL applies "background" water damage to the player's inventory and items on the floor (just like with fire in the ToEF, or shock in the Air Temple), whereas rivers do not affect items placed on their squares, and obviously only damage the player's inventory if he/she swims through them.
Swimming across a river with fluff balls in the missile slot (or anywhere in your inventory, if you have no waterproof blanket) works correctly. Dropping them in a water tile does not.
Swimming through the WDL with fluff balls in the missile slot works fine. Swimming through the WDL with fluff balls anywhere else in your inventory (without a waterproof blanket) does not. Dropping a fluff ball in the WDL's floor doesn't spawn gremlins until it takes the aforementioned background water damage.
Dragon's ice breath attacks don't freeze water either.
Do dragon breath attacks work the same as frost bolts? A frost bolt only freezes the square of water directly adjacent to the caster. I know if you're standing on the shore, dragon breath attacks won't freeze the water past you, but I have never checked if you're actually in the water at the time.
During my tests, a white dragon's ice breath did not freeze:
* the only water tile separating the dragon from the PC, the former being immediately adjacent to water and breathing over it
* a water tile the player was swimming on, with the dragon immediately adjacent and breathing at the player