>We could, for example, make the feather of levitation only
>function as a boat if you were actually holding it...

One thing I'd like to see in these same regards.. certain boats which acted as more 'protection' than others.

Example of some boats...

feather of levitation, you slowly 'levitate' through its boatness just over the top of the water with no protection from the water around you.
seashell ring, you just barely skate across the surface of the water.
Raft, you deal with all of the bad effects of rising and falling water, not quite so bad as the feather/seashell.

Canoe, you dodge much of the water-terrain but can still be badly effected by it, though you'd be able to move faster through water than a raft.

and when you get into the heavier war-boats, they can protect you from the horrible tsunami's and deadly effects of the western aryth and whatnot.

Sort of like an inherent boat armor class.

So, from a boat users perspective I'd hate it, but from a "Wow, that almost sounds practical" I'd love to see the ocean take its toll on people. As well as the rivers!