Awesome, that rocks. Thanks! It's working great.
Ok, now I realized that my switch logic is inverted. I actually want the valve to open with the switch is open. So I modified the circuit you suggest as follows:
Connected +5V --> 220 ohms --> base
Connected ground --> float switch --> base
This seems to work, and makes sense to me, but I wanted to make sure I'm not doing something Very Bad(tm).