when yu place a boat while under water and right click to enter it you go into ghost mode.

I'm unable to place a boat while being under water :/

Or do you mean place a boat on the water, dive under the boat (be fully under water) and then jump into the boat?

Also what you mean with 'ghost mode' ?

Can you just fly around? Can other players not see you anymore? Do you become transparant? Do you look like Caspar the Ghost? Do you stop colliding with the world?

@Grum Yes, you fall into the void and don't die...