Thanks so much for your input Suga. We have a "quick recovery" gas hot water heater. We've never run out of hot water with deep baths or long showers but obviously we've never tried to fill a birth pool before!

I like your point of the less water surface on top and it's nice to hear reassurance that the width would be fine. I think I'll stick with the rectangle- once it's full it will be nice not having to worry about the water going cold.