I'm going to handle @everyone here
The pool was for the intimate parts. You're not going to try and film the most emotional part of the movie in a giant pool. That's just for when they need all the extras and special effects.
They filmed their 1on1 in a kiddie pool in a quiet little room and thats that
And if Jack got on the door too it would've weigh it down putting it partially underwater. Rose and himself would be able to float but would be wet the entire time possibly wet enough to freeze them both even. She only survived bc she had the door to herself and was able to dry off. Even if they both could've made it being somewhat wet on the door, Rose had a much better chance of surviving if she able to be completely dry. It was a legit sacrifice he made and VERY heroic and romantic.