Temperature monitoring is actually a new feature from Alarm.com and Qolsys. In version 2.5 Qolsys added sensor group 25 for non-reporting environmental sensors. You should be able to use the PowerG PG9905 with its outdoor waterproof temperature probe. I haven’t heard of anyone using it for a spa yet but I don’t see why not.
Currently you can only get notifications when the temperature hits a certain threshold but later this year Alarm.com should be releasing a raw temperature monitoring feature so you can track temperate measurements over time in charts, similar to the energy monitoring feature. I have not yet used this new feature myself yet but we’d be happy to investigate if you have any questions about it.