Ranco thermostats can be used in water as I remember from a couple of years ago when I kept reef tanks people would use the Ranco's on their chillers to keep everything at an exact temp. Aquarists always swore by them and claimed that they were nearly bullet proof. More modern aquarium thermostats have come out since then but unless Ranco's have changed within the last two years or so I don't think you would have a problem. Check on reefcentral.com for your own peace of mind.