It varies a lot. A terrible enclosure at a terrible/ fairly poor zoo may not even be deep enough for the hippos to completely submerge, while another zoo may have a large water space[Longleat's hippo enclosure has an entire lake for them!].
Generally though, a good enclosure will typically have a water space deep enough for the hippos to properly submerge.
If you want more info, this is a good thread: Average depth of a Hippo pool.