Plot hole: Gilligan admits he put the Professor's measuring stick in deeper and deeper waters to catch bigger lobster. So wouldn't the Professor have noticed the stick has been moved further out (and more than once too) in the water rather than think the island is sinking? He always had all positions and locations down to a science in many episodes, so how could this have gotten past him?
Revealing mistake: As the Professor and Skipper get the test vehicle rocking faster, there are a few times that the metal poles built into the table legs to slide it is visible. Then as Gilligan slides on the table, the channels the poles slid along are visible, and it's obvious that the figure flying out into the men's arms is a dummy.
Answer: Hungarian-to-English translation aside, Erika's log-book entries were utterly meaningless. When the radio interviewer expresses confusion, Erika even reads entries from the log: "You take a left at a big, beautiful, pink tropical flower, then pull over and park," and "After the storm, we backed up and made a U-turn," etc. Her directions were scatterbrained, to put it nicely. Additionally, Erika's yacht was forced to leave the island during a tropical storm, and they lost their bearings for several days before the Navy found them. Given that Erika was such a scatterbrain, we might also assume that she didn't hire the most competent yacht crew, either.
Charles Austin Miller