When the trapdoor is put it in the same height of a block and they are underwater, you can't open the trapdoor if you are a lot close the same, only when you are far enough to have the option to open. Also, if you try to break the trapdoor, you cannot do this, because you completely ignore it when you try to break.

Sorry for any grammatical mistake, english isn't my first language.

https://www.youtube.com/watch?v=6RvR5n929iE