Boat create visual bugs under conditions

Hello, while I was playing on my flat world, I noticed a problem, I was on slabs with my boat under some blocs and I saw some black cubes taking a lot of space on my screen.
I think it has never been caught because no one goes on slabs with a boat.

 

I am sending you a vue in 1st and 3rd person with a reconstruction of how you can reproduct the bug (the water don't do this bug)