https://www.youtube.com/watch?v=UNSMBcxUhC4 Here's a video of the issue.

The ground is flat and there's nothing underneath. It is underground, but I've been able to create underground pools before with no issue. Yet this one, the curser won't even lay on the ground half the time!

I've been able to make underground pools before, as I said, and haven't had any issues.

Maybe you could try building a floor below the floor where you want the pool to be (of course leave it empty) and then go back and try to build the pool. Or just exit the game, load again and give it a try. Sometimes moving your house to a different lot (or to the same, for that matter) helps to solve random issues.

Oh, this isn't a house. I'm making a lot for a "hotel" of sorts. Just an open lot but I figure I'd call it a hotel. Heck, I'll make more money off of it than off of my resort.

Could it be because it's on an island? I don't know what's underneath it. But I was able to build a fountain in the "lobby" no issue.

I wish we had some high-level coders who could get into the depths and find the codes.