I learned that, when you have a raft, it's important to start trying to make a raft.
They needed a raft made of logs at the end for "proof" that the island existed.
Have to think they just couldn't afford the cost of damaging the rubber dingy they had, so just went ahead and ignored the fact that they were carrying one around on the beach...
But then why not just have them wash up on the beach without the rubber dingy, like most other movies about stranded people have done? They couldn't have filmed it before the part about needing a log raft was written, since they start talking about building a raft as soon as they hit the beach.
That's what makes these types of movies so special, the complete lack of common sense while making them.