The Water Cave is another little addendum to the Maze of Caves, and therefore like the rest of the caves there's a good chance the players will completely miss it.
If they do find it, though, they'll come across a cute little piece of design.
The setup is this: players entering the room find a natural cavern filled with "a stagnant pool of brackish water". In the centre of the pool is a small island covered with bones, "spilled coins, and other small objects". That's D&D-speak for "loot". No obvious danger presents itself.
Players aren't idiots, and will suspect a trap. They'll probably suspect the trap to be the water, which they will imagine to contain acid or sharks or somesuch. Crafty Eladrin players will note that the distance from the entrance to the island is exactly five squares, and may be tempted to use their fey step to teleport across the water. Other characters will note that the distance from the shore to the island is ten feet, a relatively easy running jump, and try leaping.
Once one or more characters have reached the island, the real threat emerges; a blue slime, which has been hiding under the water. The slime's a tough competitor, with both standard attacks and area effects that can really mess a character up. Characters isolated on the island will find themselves facing the slime alone, without the benefit of a running start to jump back to safety. The tactical challenge for other players involves either fighting the slime at range, or attempting to join their allies on the island.
I really like the way that this encounter runs the lure of the treasure. By being exactly the right distance away, it tempts players into using the jump and fey step mechanics to their own detriment. The obviously unpleasant water counteracts the room's overly-suspicious atmosphere and tricks players into thinking they know what the threat is without being unfair about it.
What I don't like is that this is the second ooze in a relatively small set of caves. The emergence of the slime should be a moment of surprise and horror but players who've just killed the ochre jelly will more likely be saying, "What, another one?"
Incidentally, this encounter features a half-page "history" sidebar explaining why there is a blue slime in this room. That's more text than all the NPCs in Winterhaven get combined, it's a quarter of a page more than the final villain Kalarel gets, and it's the only room in the entire keep that gets any "history" notes whatsoever. Strange priorities, indeed.