From the international boat standard ISO 10133:
4.5 All d.c. equipment shall function over a voltage range at the battery terminals as follows:
- for a 12 volt system: 10.5 V to 15.5 V;
- for a 24 volt system: 21 V to 31 V.
4 * 3.7 V = 14.8 V So, why worry?
Normally a boat need about 30 kW/1000 kg to plane. So if your boat with grew weights about 120 kg, you need about 4 kW of shaft power. The wider the board the better.