I spent a bit of time in the ferry game and worked sometimes with a guy who has been in the water 4 times. Twice in singles and twice in twins. The twins were both engine failures. One was a Chieftain and the other a BN2. He told me that he was able to fly the Chieftain in ground effect but the remaining engine was running so hot that he was afraid it would seize. So he elected to put it down while he still had enough control to select the wave of choice. I don't know why the Islander wouldn't fly, but guess it was also a weight issue and I don't suppose ground effect would do much for one of those anyway. From memory, the singles ran out of fuel. Back in the days before GPS when they had no real idea of the winds, running out of juice was just another part of the risk.
Back on thread - our man did have GPS, so he knew perfectly well where he was and presumably how much fuel he had left for a diversion. What he seemed to fail to compute, was the need for a diversion.