How do I match a battery pack voltage to my solar panels?
If the 18 V rating on the cells is the zero current voltage (open circuit voltage) it may not be high enough. Since the cells have some internal resistance, they will be at a lower voltage when there is a load. You may want to consider more than the batteries for the load since you may want to operate the boat and charge the batteries at the same time. If you need a boat that can function no matter what the solar profile, you need to make sure that you can get your batteries charged when conditions allow it, so you need to get as much current out of the solar cells as you can even when you batteries are near full. Some charge controllers have buck/boost converters so you don't have to worry about voltage matching. This allows you to pull as much current as possible, even if it means dropping the voltage on the panels down to 3 or 4 volts. If you are building your own, maybe you don't have this option. In this case, you need to open circuit potential to be as high as possible to get high currents at you battery voltage.