Desert Island is a sequel to The Indian Show with Lucy being terrified of the "giant native" who she presumes wants to eat her.
However, there is something much more sinister at work.
The whole problem begins when Lucy concocts a plan to briefly strand all of them in the middle of the ocean so Ricky and Fred will miss a swimsuit contest. Lucy gets the dock master to only fill the tank halfway so they'll run out of fuel unexpectedly. She then also has a thermos filled with gasoline she can pull out once she's sure the contest has been missed.
While preparing, Ricky and Fred discover the thermos. Ricky comments that is it very dangerous to store gasoline in a container intended for lemonade. So what does he do? He takes the thermos and just leaves it on the dock. A container of flammamble liquid just abandoned on a public dock without any indication of its hazardous nature.
If that happened today, it would look like a terrorist act.