Fire belly toad water ares is waterfall?

[Sunshine]

maybe, but add an extra thimble just to be on the safe side.

p.s. have you thought about keeping something else instead i.e. a soft toy frog?

That was unnecessary.
Anyhoo, I would say no to just the waterfall. These toads are semi aquatic so they will need a large enough body of water that they can swim. If you are looking for an easy setup you can fill the tank with a few inches of water and some large rocks to climb on. Here is a pic i found. Sorry it is so small. You get the idea though.