So
Lifer
I was thinking (after reading this article) that if you are worried about drowning being too cruel to mice, why not drop a bit of dry ice in the bottom of the trap. Let it sublimate and you'll get a bucket of CO2, with no air. Mouse falls in and suffocates in about a minute. The only downside I see is that dry ice isn't as cheap as water, and you'll probably have to refill it periodically.