There are ways, but eahc has it's own ups and downs. The way I would've liked to have done it is by making all the water "ford" and then turning into a boat when you go on it, but by making all the water ford texture, the map fails, so that's a no no. The other way would be by using steppe, but then the water meshes with the land.
Or, I could make it so that you have to pay for a ferry from a city or waterside castle, but I can't script that, though it may be done in the future.