How should I best write in an "out" for this obstacle if my players just don't get it?

I read one time that a good way to do obstacles is to put the impediment in the players' way but not have a specific way of them getting past it and to just try to go with whatever they came up with. So here's what I have for them to encounter:

They're cave crawling and they come across a small pool that runs perpendicular to the tunnel. They can see the tunnel continue on after the pool, but it's thirty feet away. The pathway ends ten feet above the water which is 100 ft deep and connects to several other points in the cave (the players don't know this). The water is frigid and players will get exhaustion points if they attempt to swim in it and fail CON saves. Once across, players will find a lever behind some rocks. This lever will raise a small bridge across the pond. One idea I had was that they could build a small walkway with the rails from the mine carts by tying them together but I figured I'd really just say 'yes' to any plan they had that seems reasonable. Party consists of a Ranger/Rogue, Cleric, and Fighter.

My concern is that they just won't come up with anything. Sometimes the party can be somewhat unimaginative but they need to cross here to continue in the cave. So what are my options as DM if the players just don't get it?