Idk why you’re being downvoted. They did, and you’re right. A door of that size made with accurate wood could hold two adult men out of the water when they removed their life vests and put them under the door.
If Jack and Rose had done the same, they would have increased the chance of survival for both of them by both being out of the water and having double the body heat to warm each other.
Because you missed in the same episode during that part where David Cameron said that Leo’s character dying was what had to happen. That was the story. It is a pointless gotcha, based on the movie not spending the time to determine the exact amount of buoyancy of a ship wreck.
They try in the movie and it causes them both to be in the freezing water. It wasn’t about him being able to float, it was about them both not freezing to death.