The boat can make players phase through nether roof.

To reproduce

1. Create a 2x2 area below the nether roof.
2. Place a boat inside the area.
3. Mount the boat.
4. Dismount the boat.
5. Tries to jump, if fails to reach the roof, that place a boat above the nether roof and mount it.

Expect result

You would not able to phase through nether roof.

Expect result

You can phase through nether roof.