RE: wiring batteries
it would work fine. and your pump would last 3 times as long. lol. as long as you know how to wire them in parallell. i do it all the time in my RC boat, i have the equivalent of 43.2v on my r/c boat, with that running in series my motors would spin 60,000rpms for about a nano second, then my speed control would melt right out of the box and through the hull, sinking my boat. lol. but since they are wired in paralell the motors(built for 14.4) run half speed, enough to plane out, and my setup will last almost an hour at full throttle, vs. 8 minutes on 14.4v.