I have a Seaven Seas hot tub with a Brett-Aqualine control
The temp went up to 115 without anyone touching thermostat

You didn't give us any model numbers on this Brett System to go on.

But in absence of this.... (This assumes an electro-mechanical system, not electronic).

Rotate the thermostat and see if you hear a clicking noise that will signal the switching action of the switch. If you hear it... then it might be ok. If not, replace it.

If you have a topside control with a thermostat then you need to watch the "Ready" and "Heat" lights when you rotate the thermostat through it's range. If the lights don't change, then replace the topside control - (same thing as replace the thermostat).

Lastly, if the thermostat is clicking and/or indicating a change from ready to heating.. then there will be a relay turning on or off with it, and you should be able to hear it. If not, then the relay or contactor is probably stuck in the on position, and needs to be replaced.

You'll really need to get into this thing to figure out what's going on. If your system is electronic, then everything changes.

Let me know, and we can move beyond this point.