Revealing mistake: Almost every time Sam looks in a mirror or the viewer sees Sam in a mirror, the movements of Sam and Sam's "reflection" hardly ever match up correctly as if it were Sam's real reflection.
Revealing mistake: Al is supposed to be a hologram. However, he is constantly casting shadows, and as a projection, he shouldn't.
Factual error: Many of the episodes are set in the 1950's or early 1960's, but we always see three-prong electrical outlets in the buildings. Three prong electrical outlets only came into common use in the mid to late 1960's, and were required starting in 1969. While they existed before that, they were only installed in specific use situations.
Answer: Sam threw items off the raft to lighten the load, so it wouldn't sink, in doing so he made the raft less heavy. Which made easier to float with the currents, if it was heavy the raft would have moved slower and not moved so far.
Except that Al said that immediately after throwing stuff out of the boat four hours passed and it was shown that they didn't really move from where they were. They were still in the same spot. Forgot to ask this too. When Sam and Vanessa are stranded on the island, one of them, can't remember who, did something and when Al appears, he tells them that because of it, time had suddenly skipped several more hours and if the event hadn't been interrupted, they would have been rescued by a boat. So, what happened on the island that once again caused time to speed up? It seems kind of strange that time could move so quickly on the island, especially since it was still day time and it never showed any sort of changes like the sun or clouds moving.