Fuel of Choice
For convenience, choose a genset that operates on the same fuel as your boat. For example, with gasoline inboards, select a gasoline genset so you can tap the main tank.
There is one possible exception. On an outboard fishing boat, which runs on gas, it's sometimes wise to have a diesel generator. Outboard boats rarely feature ventilation for below-deck compartments. Diesel (less volatile than gas) is safer to use in such enclosed spaces. You'll need a separate diesel tank, so account for that when evaluating available space. If adding a genset, you'll also need to customize a system to usher air to the generator.