The hole isn't as bottomless as it looks, it turns out. The bolt of light travels just a few feet before hitting water. The surface ripples where the spell hit it, but nothing else happens.

The hole aligns with one slightly slanted, slippery-looking wall. It doesn't look like it would provide much of a grip, but it might be climbable with some assistance from above or below. That's about all they can see without poking their heads in.