Before purchasing a heater you should verify for certain that your panel is wired with a large enough wire to handle that kind of load. It sounds like it is, but its not the kind of thing you want to find out the hard way.
I would respectfully disagree that your current panel subpanel setup is adequate. We don't know that yet. You need to determine current main panel load and subpanel capacity and load. If you are going to use the subpanel you need to make sure the wire size from the main to the sub is adequate. As Danny above points out that means a minimum 30 amp dual pole breaker.
As stated, the main panel uses a 100-amp dual pole breaker for the line to the sub. I have not verified the actual wire itself and am not certain how I would go about doing that. The sub currently only powers our 1 hp pump on a 20 amp dual pole and the pool light on a 15 amp single pole. I'd have to figure out the load for the main breaker but my current assumption (yes we all know about assumptions) is that it can handle whatever the sub panel can. It appears to be designed with heavy use in mind due to the 100 amp dual pole breaker, and our main panel currently has several breakers that aren't even being used including one for a water softener and one for a garbage disposal so it's not using everything it was installed / designed for.