In reality, it's probably a lot less likely that either of them would have survived had Jack climbed onto the door. Whilst it looks like the door is sturdy enough to keep them both above water, they never actually try and so we cannot definitely say that it could hold the added weight that Jack would bring. Even if the door had even dipped just slightly, the freezing cold water would likely have risen above the edges, leaving them lying in the same freezing temperatures that killed Jack whilst they waited to be rescued, not ideal.
Really, when you think about it, even if the door does look big enough for them both to lie on, that is only from our perspective. Jack is not only rapidly freezing to death, but also clinging on to the side of the door. His view would have been stunted to say the least and he couldn't possibly know that the door would consistently hold his weight. Even if he thought they would be fine, there is a good chance they would have both died if we were wrong, something he was clearly not willing to risk. In short, Jack ensured Rose's survival 100%, regardless of what it meant for him, probably a more romantic ending to them both surviving anyway.
I'm a Scriptwriting degree holder from Bournemouth University and spend most of my days furthering my extensive passion for Film, TV, Music and Videogames. I am an unashamed geek and have a tattoo of the Dark Knight Batman symbol on my back, also love a good story and highly look forward to when Liverpool FC remember how to play football. Follow me on twitter for more random media related musings @GuyWidBatTattoo.See more from Adam