Most inline heater housings (to retrofit a standard heater) do not have a fail-safe switch. I am not sure about the models sold as purpose built in-line heaters. In any case, you want to place the heater in a position such that it will not be able to run dry, even with the return or heater pump turned off. If you keep that in mind and set the heater up approproiatly you will be fine.
As always, you really should be using a dedicated temperature controller or reef controller to manage the heater instead of relying on its built in (failure prone) thermostat.
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.