Expectation

The expected outcome of leaving a boat is the capacity to play normally.

What really happened

Upon exiting a boat, my selected item did not show up in  Steve's hand in first person mode. Accompanying this, I discovered that I was not able to exercise any control over the world except for breaking blocks which functioned normally (blocks drop what is appropriate for what the player has equipped). Mobs cannot be attacked in this state; however, the sword icon does appear below the cursor when looking at a mob.

Right clicking loses all function in this state. That is, right clicking with food does nothing,  right clicking a crafting table does nothing, and blocks cannot be placed. It is not even possible to get back into the boat. Right clicking in the inventory functions as normal, and the 2x2 crafting grid works normally. The only way to escape this state that I have found is to quit to title from the escape menu.

Replication

1. Enter a boat (experimenting in creative showed that any type works).
2. Press the shift key to exit the boat while pressing any movement key (i.e. wasd).
3. Upon exiting in this manner, you will be incapable of anything aside from movement and breaking blocks as described above.