Hi, as a homebirth midwife I attend waterbirths and what I mostly recommend is this:
In early labour fill the pool halfways with quite hot water and put a cover on it. If you need the pool soon thereafter, simply fill up with cool water to achieve desired temperature. If you only need the pool much later on when the water has cooled down, fill up the pool with hot water to achieve desired temperature. Blow-up pools generally keep the temperature very well, especially when covered.
If water gets too cold while you are in it, just remove a few buckets of the water and add hot one. This works really well, no need to get a heater.
I've mostly worked with the blow-up pools from The Gentle Birth Company called Pool in a Box and they are really good. They also sell a blow-up cover for them.
A word on water temperature for births: It should always be what is most comfortable for the birthing mom. Some like it cool, some like it hot - it's all fine!