Rick,
Batteries have two ratings, voltage (v) and current capacity (mAh). Most rechargable batteries (except 'Lipo's) are rated at 1.2 volts per cell, at whatever capacity. Higher voltages usually mean higher speeds for a particular motor till you get to the 'fry it' point (which can be different for each motor size). So more cells in a battery pack means higher speed, to a point.
The capacity, the number of mAh (mili-amp-hours), of a battery corresponds to the size of a fuel tank, bigger means a longer run time. One mAh is 1/1000th of one Ah. So 3000 mAh means 3 Ah. If your motor draws 1 amp per hour (max current draw) then it should run for about 3 hours on one 3000 mAh battery (always figure on not running quite that long because of circuit resistance and other current draws in the system). A 'fudge factor' (technical term) factored into the equation will put you closer to what you will actually get out of the battery. A fairly good general type 'fudge factor' is about 90 - 95 percent. So if the numbers say it ought to run for an hour, count on something closer to 50 minutes (probably get more, but would rather bring it to shore to soon than to late, right?). A battery's capacity, it's mAh rating, contributes nothing to speed, only duration. That's not exactly true since higher capacity batteries usually weigh more than smaller capcity batteries.
Motor manufacturors always tell you the voltage that their motors are rated for. That maximum voltage always has a built-in 'fudge factor' by the manufacturor. Different manufacturors use different 'fudge factors', so there really isn't a 'standard' one. They do that to keep people from burning up their motors. So, most motors can be 'over-volted' to some extent without generating smoke. A volt or two probably won't make any/much difference, and you can tell by the heat generated by the motor (and ESC! and battery!). How much you are willing to bet determines how far ~you~ can push a motor. If you are not willing to bet the price of a completely new boat, I'd be really careful about gamboling (pun intended).
Hope that helps... "But what do I know?"... sound familiar? - lol
- 'Doc

About the 'lopos', I'm totally clueless about them. The same things still hold true about the voltage and current capacities though. If the voltage stays below the motor's 'fry it' level it ought to work just fine. 6000 mAh or 60,000 mAh capacity, only difference is in the size and weight of the battery. It's still comparable to how much fuel the tank will hold, not really a 'biggy' (unless you're paying for it, right?)...