No expert here, but I have an EasyTouch8 (ET) and also run Insteon. I've not considered mixing the two because the ET seems to just do a very nice job and it has schedules capabilities. I'll qualify that by saying that the part that makes it very easy is the Screen Logic interface (also from Pentair for iOS and Android) for the EasyTouch. It's a few hundred bucks if your ET isn't too old - just something to consider looking at if you don't mind spending more money. You can then control everything from your phone, but it is still not integrated with your Insteon.
On to what you have asked about though - Insteon makes some low voltage controls for drapes, sprinkler systems, etc. You might be able to integrate those to control part of the pool (use low voltage to trigger the relays), but I'm not aware of how to get temperature readings back out of the ET to the Insteon system for further logic decisions and actions. The mother board on the ET can take logic commands from an outside device (it does it from the Screen Logic and Pool side or indoor mounted remotes) - you'd have to know and decipher those signals - and then learn how to make Insteon duplicate those signals for you. I've not seen anything around that would do that out of the box.
What Insteon controller do you use? I use the ISY device from Universal devices and really use it just for lighting control. I've got about 40 switches through the house, fully automated for dusk, dawn, etc. I say this because as much as I like automation I'm just OK with each being tailored to what it does best and have not yearned for a single place to control it all. The Screen Logic is so simple and straight forward I can't imagine anyone else doing it better, I really like it's ease of use. I've got a Honeywell thermostat, and just use schedules and it's independent phone interface (I already gave up on Nest). Same with my alarm and music system - I just use their dedicated app. I just keep all of my apps in one folder and use them for what each is designed to do best. Would it be cool to hit one button and turn the spa on, adjust the temp inside and turn on music outside - yes would be very cool, but I haven't seen anyone control it all and do it well. The Amazon Echo (has an interface for Insteon) and Homekit I guess are trying, but nothing is mature enough.