From what I have read, some BPs like to soak, but most do not, and sometimes it can be a sign of mites. If you put in a waterfall and pool, it will likely be more for your enjoyment than your snakes. Be sure that it does not raise the humidity TOO high, and that it is not keeping your substrate wet, which can lead to belly rot.
As for building a waterfall/bathing pool, you can use a whisper 10i, which is a filter and a pump, with easily changeable cartridges. you can also substitute your own filter media rather than buying the cartridges they make for it (it is cheaper to maintain that way).
I'm really not sure about fish... You could also ask this question at:
BP Forum. They are very knowledgeable over there.