Well,
You came up with a solution that works for you, and that's good..
I view this situation as being on the edge of a storytelling versus simulationism question. I'm not saying that you fall into either catagory, but the players seemingly 'over-thought' the situation from your perspective. That may not be strictly your fault, or theirs for that matter. Players do odd things sometimes, because of the way they've been conditioned.
For example, they might have looked at this situation and thought "the moment we turn back, this will be the time that we miss out on some cool treasure or a secret chamber."
Perhaps they've heard a GM pull this on them one too many times:
Because some GMs (and I'm not saying you, I'm speaking in generalities), pull the
"damned if you do, damned if you don't" too often, and players cease to trust their instincts or even a gentle hint. 'Cause, everything is a fake out. Inexperienced GMs (again, not you) can scar players by following that adverserial GMing style.
I don't think you did anything wrong, your players either didn't notice the hint or they second guessed themselves.
I'd ask them, now that you have a resolution, if they sort of heard the hint and decided to try it anyway. Because if they did, that's a trust issue. They're trained by some experience that DMs are "tricksy".
If they reply that they didn't hear the hint in your words, then maybe they weren't paying attention.. or you might have been just a little subtle for their level of play.
The only thing I might have done is ask yourself, 'is it going to affect my game, and my own fun, if they just drown?' The answer to that question might mitigate how much of a stronger hint you want to give next time.