I don't think it's worth it. The extra weight in the hull and more importantly extended runtime isn't always good in boating. The electronics are abused much more then in land and air vehicles and the results is more heat. With a mild prop such as a graupner CF42 and those 5000mah turnigy packs you should see 4 or 5 minutes of good solid runtime.  I think that's plenty of runtime. Just remember to drive properly (constant wide open throttle, and long straights) and allow cooling in between battery packs.