We LOVE our Prius. At 50-53 MPG, it is unreal (not a car, but a life-style). They have found that the battery pack lasts a lot longer than first expected (they haven't been failing, well beyond the initial 8 yr. warranty). To say that there is no help to the environment over a car getting 1/2 (or less) that in mileage doesn't make sense.
That said, I wonder why the boat manufactures don't make a hybrid boat? So, it could run on a battery when idling, and the gas engine when pushed over 1k rpm? In our sport, I believe the gas savings would be enormous.
Idling plays a very big part in boating, in fact, many boats spend MORE time idling, than running at high rpms.