Precisely what happened in one of my games. I was a player, and we managed to sniff out where one of the bad bosses lived, and walked in for a "fair fight." I don't want to go into details because (a) I don't want to put the DM on trial here and (b) we, the players, didn't try many of the things I covered in this thread, so we have ourselves to blame.
Was it a TPK?
The one time I had a TPK was at 3rd level. The PCs were hunting goblins in a forest, trying to rescue homesteaders who had been taken prisoner by goblins. In an earlier encounter in the forest, they had been attacked by a deranged homesteader, driven mad by his experiences, and some of the PCs had killed him, triggering recriminations from the other PCs.
Thus, when I described a group of homesteaders huddled around a fire, the players, feeling a little burned by that earlier experience, were predisposed to have their PCs respond with sympathy rather than suspicion. Which meant that the PCs were all seated around the fire in a nice position for my spectre's aura 1 auto-daze, while the "homesteaders" revealed themselves to be pale reavers (from Open Grave) cloaked in illusions.
The players quickly worked out that they were in trouble, but the action denial was utterly brutal - more brutal than I had anticipated in designing the encounter, in part because I hadn't expected the players to be so un-suspicious, because I hadn't anticipated the way this encounter would bookend against the earlier one with the deranged homesteader. So the PCs all went down. But the only one who was confirmed dead (as opposed to unconscious) was the paladin, who was dropped below negative bloodied by some friendly fire from the PC wizard.
The way I handled it was, after the session, to send around an email clarifying who wanted to keep playing their PC, and who wanted to change. Only one player wanted to change. So at the start of the next session, 3 of the PCs regained consciousness in the goblin's prison cells. Imprisoned with them was a strange dark elf (the new PC). They could smell the smell of spit-roasted half-elf (the drow player's old PC). Meanwhile the paladin, whose body was splayed out on the goblin shaman's altar in another part of their goblin caves, was sent back into this world by the Raven Queen at a crucial point in a subsequent encounter, in a way that built on some earlier stuff that had happened in the campaign involving undead and witchcraft.
I'm not entirely sure what I would have done had the Hydra fight TPKed, but I think I would have tried to use the duergar somehow, and also - if the PCs souls had ended up in the Shadowfell - would have drawn in some fashion on their earlier
pledge of allegiance to Kas.
I have the opposite problem with my players. They never ask how tough a monster is. Of course I'll describe, and try to warn them if they are in trouble, but they frankly don't care. Even after I've killed them a combined 40 odd times in this campaign, if they see a monster, they're going to try to kill it, regardless of whether it's a lone kobold or two dozen water trolls splashing down a river
Do the players mind having their PCs killed?
Also, with situations like the water troll one that you describe, as a GM what role do you envisage that playing in the game? And how do you communicate that to your players?
(My reason for asking these questions goes back to my earlier comment about the importance of knowing one's GM.)