| Are you serious? Can you comprehend WEIGHT of battery VS number of cells? | LiPos have far greater density of power so you can get higher voltage for | the same weight.
Voltage doesn't really matter. It's power that matters. I'm sure you already know this, but I'll explicitly state it for those who don't realize it.
What you ought to be looking at are four ratios :
capacity/weight power/weight cost/capacity cost/power
(You could think of a few more ratios, like cost/weight, but who really cares about that?)
I mention ratios rather just capacity or just power because you can always double your power or capacity, by doubling the number of batteries, but you're also doubling the weight when you do that.
LiPoly *definately* have the edge over NiMH and NiCd in capacity/weight. This is where they shine.
As for power/weight, I believe they have the edge there as well, though I'm not certain that there aren't some NiCds that can actually dump more power per weight than the best LiPoly cells. It could be the best 90 seconds of your life! :)
As for cost/capacity and cost/power, LiPoly cells seem to be a little more expensive than NiCd and NiMH cells, at least for now. As time goes on, I expect this to change.
So what could LiPoly gain you in your boat?
Assuming you want the same weight in batteries, they can get you either more power, or longer durations, or some combination of the two. Or it could get you the same power, same capacity but less weight (which is less important in a boat than in a plane, but it's still nice.)
Of course, this is all based on the assumption that you make appropiate choices in deciding the correct types of cell, pack arrangement, the right gear ratio and the right props.