have you tried turning up your water heater? sometimes this helps.
I dont' know that I'd use anything with the tub - especially a tub that has been used before. That might compromise the integrity of the seams. Those inflatable pools are not really meant to be used with waterbed heaters and the like. Heck, I think they're totally awesome for standing up to using them with hot water for birth!
was the water not hot enough for your liking or was your provider concerned about the temp? sometimes women don't mind a bit cooler, but there are still many providers who think the water has to be hotter than 100'.