Steps to reproduce:
1. Have a couple of different blocks on your hotbar, then get in a boat
2. Face towards a block on the world, then try to switch hotbar items
You cannot switch hotbar items. It appears as though the player is trying to place blocks but can't, and the hotbar switching fails.
The player should be able to switch items while riding a boat.
(It might be worth noting that I'm on mobile using touch controls)