Diesel/electric drives are nothing new - many ships and pretty well all locomotives have used this idea for decades. As an auxiliary it's attractive from a noise/vibration perspective but realisitically to produce as much power as a conventional diesel drive the generator will need to be the same size or larger. So you won't get away with a smaller "cheaper" engine this way. Depending on battery reserves will help, but ultimately there is no free lunch when energy is concerned. Packing huge (read heavy) battery capacity is going to hurt in other ways, perhaps even more so with multihulls.
Maybe the fuel cell idea will come to a more practical level of technology in the nearer future.