I was moving villagers by boats when I ran into this issue, I got stuck on grass between path blocks and decided to use my hoe to get over that grass.

What I expected to happen was...:
I expected I could just go over that farmland, and that worked fine. The problem is when the farmland changed back to dirt. I don't really know what I did expect when that would happen, but I guess I'd expect the boat to break.

What actually happened was...:
It looked like I got kicked from the boat, but I wasn't really.
Upon further investigation I came to the conclusion you can't do anything when this happens, just walk around. You're totally invulnerable to everything, except for anything that happens where the boat is. 
I also tried this on a LAN-world and vanilla server, same thing happens, but it appears it's only client side. For other users (and also mobs that try to attack you) it looks like you're just in the boat instead of walking around.

Steps to reproduce:
1. Use any hoe on grass or dirt, to make it easy make it a 2 by 2 square
2. Place any boat on the farmland
(You can also place the boat first and then use your hoe, it doesn't make a difference)
3. Wait until the farmland turns into dirt
4. Ta-dah, there is your bug. You can walk and look where you want, but can't interact with anything. 
5. Press your crouch button to get out of this bug, you'll be teleported back to the boat, as if you got out of the boat (which you actually did)

Additional notes:

• I thought this would be exploitable in some way, though I'm not sure how, that's why I marked this as private. If it isn't, feel free to make it public.
• Also tried this on 1.16 and 1.15.2. Boat breaks on 1.15.2, same bug happens on 1.16.