The Daily Beastwriter Marlow Stern wondered about this Titanic plot hole as well. So, during a recent interview with director James Cameron, he asked if Leo could have fit on the plank. And Cameron responded.
"Let’s really play that out,” Cameron said. “You’re Jack, you’re in water that’s 28 degrees, your brain is starting to get hypothermia. Mythbusters asks you to now go take off your life vest, take hers off, swim underneath this thing, attach it in some way that it won’t just wash out two minutes later—which means you’re underwater tying this thing on in 28-degree water, and that’s going to take you five to ten minutes. So by the time you come back up you’re already dead. So that wouldn’t work.”
Cameron also added, “His best choice was to keep his upper body out of the water and hope to get pulled out by a boat or something before he died.” Clearly, Cameron has thought about this issue even more than the fans.
However, what Cameron didn't mention was that Rose and Jack did try to both fit on the plank at first, but it flipped over. So even without his explanation, it should be clear that it was only able to support one person, even if it technically had the surface area for two.
So if you've been mad at Rose for all these years, you can now let it go, because there is nothing she could have done to save Jack. And because there's really no need to be mad at a fictional character.