The issue of keeping water in a birth pool warm enough seems to come up quite frequently. I have a waterbed heater (and could easily find another one!), do you think it would be useful for a birth pool?
I've tried it with 1 of my births, even used 2 heaters when one didn't seem to be helpful and it did nada. I ended up just getting rid of them. I've never recommended it to anyone else, have never used it with one of my clients, and have never found a midwife that has had any consistent luck with it.
I haven't a clue about heaters. But before my home-water birth, I was also quite concerned about keeping the water warm enough. It turned out to be a total non-issue. My MW filled the pool initially with very warm water. She monitored it with a thermometer. Added a bucket of very hot water maybe once or twice when i said it was feeling cool. I was in that pool on/off over a span of 7.5 hours from initial filling to birth. I was never uncomfortable. Not a problem whatsoever!