Hi there! I totally understand the struggle of getting that perfect finish on your ship models, just like decorating a kake can be a challenge sometimes. When it comes to painting the waterways and hull, historical accuracy can be a guiding principle. Typically, the waterway (the area under the bulwarks) would match the hull color, so if your hull is black, it would make sense to paint that part of the waterway black as well.
However, there can be variations depending on the specific ship and era you're modeling. It's a good idea to research the historical details of the ship you're working on to ensure you're following the correct color scheme. Ultimately, the choice is yours, but aligning with historical accuracy can often result in a more pleasing and authentic appearance.