i've done more research, there are some good submersible titanium heaters resistant to saltwater corrosion. they're pretty cheap too, there's a 500w heater on
www.jehmco.com going for $US40. if electrocution is still a concern maybe you can fit some kind of waterproof 'sock' over the heater to create a barrier between it and the water.
there are many different approaches to take, i haven't finalised my decision just yet but i hope you get some ideas.