I have seen them enter the water when mine were placed in the rain chamber. The female swam under the platform in the tank, which is at the surface of the water, the water being at least 6 inches deep. She spend some time under there, but eventually she came out. So I would n't worry about any accidental drownings with them or any other treefrog for that matter. An example, gray treefrogs are very skilled swimmers. Saw one swim to the bottom of my pool, then come up when the coast was clear.

That being said, a water bowl will suffice with your frogs.