FWIW
A battery's current capacity deals with how long a motor will run. The voltage deals with how fast the motor will 'spin'. To run longer you'll need a larger current capacity battery which usually means a battery of a larger physical size. Hope you have 'room' for it.
An electric motor will usually 'work' on a fairly wide range of voltages. The higher the voltage the more heat is produced, and at some point that heat will 'get to' the motor, either burning it up, or affecting the surroundings (melting/burning stuff). The way things work, a manufacturor usually uses the smallest motor they can get by with, and 'pushes' it as much as possible (cheapest way to manufacture things). Increasing your battery voltage may not be the 'smartest' thing to do. Two batteries with the same current capacity, but one having a higher voltage, the higher voltage battery will 'go dead' sooner than the lower capacity battery. "Cain't win for loosing", right? What you're looking at is a balancing act with the 'room' in the boat for the battery, the weight of the battery, the cost of the battery(s), and the amount of run-time you gain. That's if you try to use a larger than provided battery. The easiest solution is to get 'more' batteries and swap them out while boating (recharging the low batteries in the mean time)...
- 'Doc

PS - 'Limits' is 'limits'. At some point you'll reach it...