After it was established that Jack and Rose could have easily managed to lie side by side on their makeshift raft, the question turned to one of buoyancy: Could the plank have sustained their combined weight?

If it helps, the wood was a piece of pine paneling rent from the Grand Staircase.

Give up? Someone already crunched the numbers and found that the force of buoyancy would be less than the force of gravity, meaning they would sink faster than a certain unsinkable ship.