I think what you want is a temperature controller. If a heater explodes isn't it usually because it has forgotten to shut off? So it keeps heating until it gets uber hot and BOOM! A temp controller cuts the power to the heater when the water reaches whatever temp you've set it at.
Doesn't really tell you that your heater is pooched, but might avoid the explosion.
I can tell you right now that a surge protector will likely not do anything. I have never had a heater blow up but I have had many that stuck on, which a GFIC did not help either. A temp controller would be a better idea, though I would put in a GFIC outlet just because.