I know nothing about watersnakes/queensnakes or what thier swimming and eating requirements are, but love a good housing question. I reccommend incorporating an aquarium in your vivarium. I have done this with several species and enclosures. My first was for a garter snake where I use a small piece of glass to divide off one third of a large glass aquarium. I used plaster to build a hidebox/burm up to the edge of the "pond". I stocked it regularly with goldfish and let the garter hunt as he pleased -- fascinating to watch. For a snake I caught in a pond and thought was a waternsake (turned out to me an eastern milk) I did the reverse: full goldfish aquarium with filters and all with pieces of glass set in "V" shapes creating islands for basking and drying out. All the glass gluing was done with clear silicone caulk. In later cornsnake enclosures built out of wood, I simlply placed a full tropical fish aquarium inside as part of the scenery. Snakes don't always move and an aquarium adds dynamic activity to the vivarium along with moisture. In the corn enclosure, the operation of an attached stream and waterfall created some humidity problems. Hope that gives some ideas. Have fun.