It should be pretty easy to create a
customer thermistor circuit, with a 741 op
amp for comparator. Such a simple circuit
would only give ON/OFF signal if the tub
dropped to a low enough temp, but that is
essentially what you want to do.
Then, you just send that signal to a power
flash.
Other option is to buy a wireless
thermometer setup. They often come with
hot tub sensors. I think that last one I
bought had one, but I never used it for my
hottub.
Rob