Doors do not create an air space to breathe underwater

You can no longer use that method underwater because of the new water mechanics. Placing non-full blocks underwater makes them 'waterlogged' meaning there will still be water in that block. This probably works as intended.

This is currently working as intended. Please visit the feedback site if you want to suggest a change.
A work around for builds or just creating air pockets is to first make a 3 by 3 by 3 cube of some solid block under water and then breaking out a 1 by 2 by 2 section into the middle. place the door and then remove the rest of the blocks. The door should no longer be water logged as water still won't flown into it.

On java it still works with the new water mechanics and they're the same.? Ok thanks I'll try that. You cannot go into a ruin or sunken ship unless you have potions or a turtle helmet without almost drowning . Why would they have this still working on java and not bedrock pe?

Its the same as java. The players do what Dr.Awesome4333 said. They create an enclosed area before placing the door or at least remove the source blocks wherever they'll need air pockets for doors.

@unknown For Java Edition the behaviour you described is considered a bug, see MC-125351.

@unknown This was be resolved as WAI.