It's not a question of room; it's a question of buoyancy. Jack puts Rose on the raft, then he gets on the raft - He's not an idiot; he doesn't want to die - and then the raft sinks. So it's clear that there's really only enough buoyancy available for one person. So, he makes a decision to let her be that person.
And romance reigns once again.
If you're still not firmly in Camp No Other Way, Cameron says the Mythbusters are working on proving his buoyancy theory as we speak.