I am the owner of a spa repair company and would be glad to answer your question. Just so happens we are the Warranty service provider for Marquis Spas in Ontario, Canada.
Based on what your describing, I have a few questions:
1. Can you tell me the model of Marquis Spa?
2. Do you have a circulation pump?
Seems like your circulation pump is not moving water which would cause a "FLO" and "OH" error messages.
"Sn1" error code means one of the sensors is not functioning propery or out of balance.
I would suggest getting a repair company out to have a look as you mentioned it is getting cold at night.
Do your self a HUGE favor, go and purchase a 200Watt submersible fish tank heater. They cost about $60, if the tub ever dies, turn that thing on max and drop it in the tub. Will keep the water about 60 degrees @425 Gallons.
Much better than a $1000 leak repair bill.
Have a great night,