In the Mounts and Vehicles section of the PHB it mentions that the animal(s) pulling the vehicle determine its carrying capacity. What determines the carrying capacity of the "waterborne vehicles" listed in that section? In the case of something like a row boat, is it the person rowing the boat?
Good question. In the real world, the carrying capacity of a waterborne vessel is determined by a comparison of the vessel's mass and displacement- when the total of the former (including what the vessel is carrying) surpasses the latter, the whole shebang begins sinking. We aren't looking at the real world, however; we're looking at a simplified analog. And in this simplified analog, we see a few assumptions:
- Rowboat speed is not dictated by the strength of the rowers. Water current is actually more significant than rowing when it comes to a rowboat's speed.
- Boats can be dragged by pack animals as if they were wagons.
- Keelboats, though specifically listed as possibilities for dragging, are not given a weight. Neither are any sailing ships.
Given that information, it's reasonable to conclude that you aren't supposed to worry about carrying capacity for a boat or ship. It can carry enough weight that space is a bigger concern than the overall weight is (so, for example, you won't be carrying four horses in a two-man boat, even if the boat could take the weight). It also explains why boats are used to carry goods upstream, even though the animals doing the dragging would be working against the current- they could still pull more than with a wagon. That said, there will obviously be special cases that could make it important- could a longship handle an ancient red dragon landing on deck?- but in such cases I would argue that the answer should be dictated by what the adventure requires, not by what physics dictates. After all, physics had little say in these rules.