What I expected to happen was...:
That when I get out of the mine cart underwater it would stop and stay there.

What actually happened was...:
The minecart started disappearing and coming back. The hitbox was still there and I could still ride it and it would come back.

Steps to Reproduce:
1. Ride an unpowered minecart into a one block deep source block with rails.
2. Get out of the minecart
3. look at the minecart and see if it disappears
Note: The minecart has to be completely submerged
Note 2: For some reason It wont happen on multiplayer