Some thoughts from the playtest:
It's amazing how paranoid players can be over mundane pools of water!
Party went in, and absent any sort of rogue, sprang every trap in the fork. The kobolds, once again, really gave them a run for their money. First From the Forest tossed up his spider climb immediately upon hearing the alarm, and hid on the ceiling. Through the fog cloud, no one could see him, and he proceeded to cast
blindness spells EVERY round until he was out of 2nd level slots. Unfortunately, everyone made their save, and I targeted every character at least once. Then he breathed acid, and that really freaked them out, and so went the
resistance from acid spells. Once they figured out where he was, he was toast. The party got slowed up pretty well with a
web spell, but it's hard to stop a 9th level party, so it was just a matter of time that they were doomed. They put up a sound fight, though.
Proceeding back towards the main area, they decided the water was basically death in disguise and set about hammering some pitons into the wall to tie ropes onto. Of course, this sound could be heard through the entire dungeon, and the dragon knew they were coming at this point. All attempts at stealth were gone. The party never discovered the underwater tunnel.
The chokers in area 7 put up a decent fight, but one of the characters had a reach weapon, which proved too much for them. They fought valiently, but couldn't hide from the reach weapon very easily.
The dragon battle last MANY rounds - way more than I had expected. The elf fighter/magic user cast
fly and was able to get to the dragon, but it promptly grappled him and tossed him in the water, where the second dragon was waiting. He was grappled, but activated some boots of dimension door and got back to safety. The party knight was really stymied by his inability to really *get* to the dragon - which took full advantage of the difficult terrain to hamper his opponents. I also rolled a "1" on just about every dragon breath refresh rate, so I was breathing acid on them just about every other round. But, they managed to get some protection from acid spells on about everyone, so for most it wasn't much of a problem.
In the end, I think it was just a nearly perfectly balanced encounter. It's challenging (the fighter/magic user ended up dying when the dragon scored a crit on an AoO), but not impossible. A very difficult battle, but one that I think will challenge a seasoned party.