I get at least one call every day from someone asking me if I have pools with heaters. I don't, and I won't.
The main reason: cleaning. Heaters work by circulating the water across a heating element and there's no good way to clean inside.
We've looked into this again and again, and Steve and I just couldn't wrap our heads around any way that we could get a circulating heater clean enough for reuse. The portable ones that resemble your home water heater can't be sanitized inside, and they are mostly powered by propane and propane accessories. (Hank Hill probably wouldn't make a good member of your birth team.)
AquaDoula is one tub that has a heater and doesn't recirculate the water. The heater on that pool is only meant to maintain the temperature of warm water used to fill the pool. (Source: AquaDoula's setup guide, page three.) I also don't like the narrow edges - the inflatable pools I carry have a 10" wide side to lean against.
Here's the good news: with the inflatable pools, you don't need a heater. The air in the floor and sides acts as a good insulator and they lose heat slowly - about a degree an hour. Also, a couple of pots of boiling water will heat it back up nicely if it's cooler than you'd like.