I have a 12v pump running on a 6V battery. The battery is rated at 22ah, but the size of a 12v 8ah. A 6 hour run only pulls 4000 mah off it. So if I run it down to its full capacity, theoretically, it will run for 33 hours
Motor speed controls are cool. I am worried about complexity rearing its ugly head though. Specially since circuits dont take well to corrosion. A simpler design would be to place a resistor along the motor power line. This is a simpler speed control. Some experimentation will be needed to get the proper resistor value to get the right speed, but after that, you can dunk the whole thing and it won't fry.
/bing