Well, now that it's looking like I can have a larger tank, I'm thinking about trying a water snake. Looking at a few sources, both the Puff Faced and Chinese water snakes are between 80-90% aquatic, and would be able to be kept in a mostly water-filled aquarium, but with a basking platform (like a larger version of the newt tank I was thinking about). Here's info on both snakes kept in setups similar to what I'm describing:
http://acrochordus.com/homalopsinae/homalopsis/hcaresheet.htm
http://acrochordus.com/homalopsinae/enhydris/ecaresheet.htm
http://acrochordus.com/homalopsinae/homalopsis/hcaresheet.htm
http://acrochordus.com/homalopsinae/enhydris/ecaresheet.htm