Boats: we should have 'em. There should be more ways to cross a river than by swimming or using a bridge, and more ways to explore than going on a road trip.
Ideally, they would behave just like vehicles, so that we could install solar panels, boxes, etc, and make them our mobile bases. And just like land vehicles, we’d have variety - houseboats, speedboats, rowboats, canoes. Also, compatibility between auto parts and boat parts would be good (plutonium-powered speedboats?)
I’m no programmer, so I don’t know how hard this would be to implement. Can the vehicle code be modified to make boats, or would this be a ground-up issue for the devs?