Navigating on a boat wearing Frost Walker Enchantment frosts the water layer below the boat.

When riding a boat on an ocean wearing Frost Walker enchantment, the second level of water is frosted.

If i'm riding a boat, it shouldn't frost.

https://gfycat.com/FakeVengefulBullfrog

Worth noting this only happens if you have gone somewhere one block deep or less.
It also applies to players riding other entities, but I like the idea of using it to get horses across, so not sure that part should be considered a bug.