While I'm not using it for a marine application, I have a Ranco controller for a 1,000w heater I use in an outdoor 150 gallon tub. I usually set it up around April, shut it down towards the end of October. Except for a Ziplock box for a cover the unit sits outside during all sorts of weather, and spends the winter in my garage.
It's been going through this for about 3 years now, not a single problem. The thing is seriously overbuilt, I think being used indoors it would last longer than the heater itself.