My theory is not a pool of water but some sort of barrier. I invision a moat, of sorts, that your alliance has to get across or reach across to get to the scoreing apparatus. A moat could be pretty small and it wouldn't have to be that deepIf the moat was narrow enough, teams would have the option of driving through it, or a robot carried ramp that allows the others to cross and even HOVERCRAFT ROBOTS!!!
By the way, my theory is based purly on hope and the belief that the competitors on the teams in F.I.R.S.T. are capable of anything.
Also the battlebots idea would be amazing, and it could be modified to be more in the spirit of F.I.R.S.T.