Re: Is a blower needed?
Because you have the fuel tank in an enclosed compartment and have electrical components in the same compartment, you could be required to have ventilation. The key is whether or not the electrical components are "protected." There mere fact that there isn't an engine located in the comparment, is not the deciding factor.
I see boats all of the time with enclosed areas under the splashwell. Many times, those areas are used to located both gas tanks and batteries. To me that is a bad idea. At a minimum, if I had a boat with such an arrangement, I would either remove the covering, or install at least a passive ventilation system. This might consist of a forward facing air intake and a rearward facing exhaust. An even better arrangement would include a spark safe blower.
Here are the USCG rules:
Boats built after 4/1/1940
At least two ventilator ducts fitted with cowls or their equivalent for the purpose of properly and efficiently ventilating the bilges of every closed engine and fuel tank compartment of boats constructed or decked over after April 25, 1940, using gasoline as fuel and other fuels having a flashpoint of 110? F or less.
Boats built after 8/1/1980
At least two ventilator ducts for the purpose of efficiently ventilating every closed compartment that contains a gasoline engine and every closed compartment containing a gasoline tank, except those having permanently installed tanks vented outside the boat and containing no unprotected electrical devices. Also, engine compartments containing a gasoline engine with a cranking motor must contain power-operated exhaust blowers controllable from the instrument panel.