Hello! I am the bard in this party. It wasn't that I didn't think the hydra could swim. It was more that it was a gargantuan creature described as being so big that it had to gnaw its' own heads off to be able to move properly while the water tunnel out of the chamber started with a pool described as 'waist height' (so about 3 ft deep). As the hydra stopped at the edge of the pool and contented itself with snapping at us from the edge, I assumed it was too large to follow. Until it did. And then it was too late (with 10+ attacks vs lv8 characters, you don't get many chances to make mistakes).
Creatures placed in a chamber as guardians and then growing so large that they can no longer leave the chamber are a pretty common trope in fantasy (particularly in the case of something that grows at a supernatural rate such as a hydra). This creature was in effect a guardian beast for an important relic that would be quite harmful to the villains of the campaign, so I don't know if assuming it was trapped in the chamber (and thus unable to wander off and get itself killed by something even more dangerous) was too outrageous of an assumption to make-Particularly as the pool at the entrance was described as being quite shallow while the vast size of the beast was continually emphasized.
It was my fault for not asking more questions to clarify the exact situation, but my confusion was not based on the creature's ability to swim. I'm not a total idiot. Just, like, 90% idiot