Near the end of the movie, Kelly shows Chuck charts detailing where he was found, where the island was, and how far he drifted - how the heck did anyone figure out the last two?
Sea currents would be one major aid. Since he wasn't traveling under his own power for the most part, he'd be at the mercy of the currents. Knowing where he was picked up, they could at the very least back track along the current. But also consider that he got to the island because of a plane crash - even if the black box is unrecoverable, they could find out where the plane might have had troubles (certainly the pilots would have said as much over the radio). So there's more information available than might otherwise seem.