Hmm. Without looking at it, but remembering what a lot of water mods do (RWT included)... I imagine that actually removing RWT from the patch would be difficult and require a lot of TES5Editing. I'm talking about Worldspace edits - not fun to do and not easy, because they are so damned tedious being that there are a lot of them. That might be a lot of work, but maybe I'm wrong.
You could always try installing Purity on top of STEP and use a custom patch to resolve anything that needs resolving. I'll tell you what though - I experimented with the notion of doing what you are asking about. I tried to take the easy way out and left the "Pure Water" (one of the parent mods of Purity) edits in, allowing it to overwrite my installation of RWT and the STEP Patch. The results were less than ideal for me. I don't know if it was due to the inability of simply installing Purity afterwards and having everything be properly overwritten, or if Purity's water is just not as well done as RWT. The place I went to see the difference between the two - down from Helgen towards Riverwood at the Warrior/Thief/Mage stones overlooking the lake/river. The surface of the water did not blend well when I left Purity's water intact. On the other hand, the surface of the water looked great when I circumsized the Pure Water portions of Purity out and left it all to RWT. But all of that may just be the results of improperly overwriting - I don't know.
Edited by oqhansoloqo, 13 May 2015 - 06:22 PM.