The word that best fits the blank is stranded.
The sentence is a conditional sentence, which means that it has two possible outcomes. In this case, the two outcomes are either the ship is sunk or Sinbad is stranded on an island. The word stranded means "left alone in a place where it is difficult or impossible to get away." This is the most likely outcome, as it is unlikely that the ship would be sunk without Sinbad being stranded on an island.
Here are some other words that could fit the blank:
marooned abandoned deserted isolated alone
However, the word stranded is the most concise and accurate word to use in this context.