Diesel-electric and hybrid aren't really the same thing, unless you're talking submarines. Traditional diesel-electric is basically just an electric transmission with no storage involved. Also most cruise liners and car carriers are diesel-electric and go well over 45,000+ tons. It's not very fuel efficient, being as it uses medium speed trunk piston engines, but does have the advantage of allowing you to spread your powerplant where it's to least disadvantage from the perspective of your cargo and passenger spaces.
There have also recently been a few experiments with smaller short haul ferries that use batteries to keep the lights on and other services running without an engine running while alongside the wharf.
That said, your main point is pretty much correct. While the motors aren't a problem at all, battery storage for a week of cruising would be prohibitively expensive, heavy and bulky.