I like it, but still think it's relevant to point out some of the issues:
- NPCs would have to "learn" how to use water craft (just as they currently don't know how to ski), which might require some effort.
- NPCs would have to be able to deal with nefarious players who steal their craft (which inevitably is going to happen).
- NPCs would have to learn how to swim eventually, when craft can either flip or toss its user into the water (rapids, possibly bad weather as well).
- The code would have to if, when, and how NPCs steal player craft (it ought to be a natural thing to do for robbers and Njerps, but random villagers shouldn't steal yours when visiting their village (peacefully, that is).
- A whole new set of combat rules would have to be implemented for "naval warfare". NPCs shouldn't just passively sit and get killed by the player, but fight back or flee, and fighting back should involve dropping a paddle to be able to use a bow (and switch back afterwards, after the player has moved out of range).
- "Naval" melee definitely ought to throw combatants into the water when they fall, at least in a significant percentage of the cases.
So, not a trivial thing to implement, but definitely a nice one (and I don't think it's impossible, "just" requiring a fair bit of work).