So to my knowlage this is only a mobile bug for bedrock 

when swimming in a one block gap  the player cannot break any blocks ahead of them they can only break the block above them  I think this is because the game thinks the player is still standing. I have also noticed when the player is sneaking the hit box does not change at all as if the player was still standing up right there are a million others variations of this bug. 

I could not find any other reports of this bug (I have an iPad Pro I don’t know if this is only for mobile)