I am guessing that boats will be a special function like gliding and mounts, and thus not take up any inventory...but will take up a shortcut keybind (I'm already running out of those due to the mounts).

As for travel convenience creep, that I don't think will be an issue, cuz as you say there's not that many waterways in early maps. Plus we already have the manta mount for speedy water travel. The only difference between that and the boat appears to be that the boat can carry multiple players, and you can fish from the boat (not sure if you can fight from it...)