If you try to toggle Third Person View (a.k.a. TPV)while riding in a boat, you are unable to. This happens while you are holding down, the A and D keys. You can toggle Third Person View when you aren't holding down the A and D keys.

What I expected to happen was:
You are able to toggle TPV while riding a boat, as it is not like you are attacking entities, or placing/breaking blocks or doing other actions you are unable to do while riding a boat.

What actually happened was:
You are unable to toggle TPV while riding a boat, I think the game treats doing this action like an action you can't perform while riding a boat.

How to reproduce:
1. Create a world with cheats enabled.
2. Use the give command to give you a boat
(/give <player> minecraft:oak_boat)
3. Find a body of water.
4. Place and ride the boat with A and D keys.
5. While holding down the A and D keys, press F5 or the key you use to toggle perspective if it isn't F5. Note: You must hold both the A and D keys, you can still toggle perspective if you are only holding down one of the keys.
6. Notice how you can't switch into TPV, this is because the game treats doing this like an action you can't do while riding the boat. (because the player is using the oars, though there is no animation of it)