There are three chests that can contain a speedo: The one at the Western Island Beach, the one in Piratetown next to the water entrance in the middle of town, and the one on Pine Island at the beach near the entrance point where you meet the giant slime. Checking either of those chests after you already found a speedo in a previous one will give you gold instead. If you still got one of those chests unopened, you can try if you can get another speedo out of it, now that you do not have the first one anymore. If that doesn't work, then I'm afraid I don't know either.