Ann -
I have the Hydor ETH-201 on my 75-gallon cichlid tank and I am very pleased with it. They are designed to run only in the vertical orientation, but they have an automatic shut-off in case it runs dry (like if you turn the flow off for maintenance). It's been on the tank for maybe five years and continues to run reliably. I have not used it on a salt water tank, but Hydor claims they are for either fresh or salt water.
In fact, I have an extra one from the FW tank I used to have in my office. It's just sitting downstairs on the shelf. Are you interested? Open to selling or trading...
Oh, and to the above point, you can always plug it into a controller to protect yourself from possible heater failure.