Do I have access to electricity and artificial lights on this island? What is the latitude? (IE can I successfully grow outdoor equatorial sativa strains?). These things do factor into the strain decision.
Going with what's already been said, if you're really stranded indefinitely on the proverbial "Gilligan's Island" with no electricity or outside contacts whatsoever, and can only have one seed, you'd ideally want it to be a hermie so at least you'd have some chance of creating more seeds and maintaining the line.
Otherwise, without artificial light you couldn't maintain clones in the off season, and so when your ONE plant reached maturity and the growing season was over your plant. . .and line. . .would die! Without the ability to keep clones, one seed = one plant, and that's it.
If you thought this was likely, one thing you might do is wait until the earliest part of the season possible to plant the seed and then once it got big enough, clone the F out of it while its still in the vegetative growth period. If you could turn your one seed into say 50 or 100 vegetative plants before autumn flowering, then at least your ONE harvest might be pretty big to tide you over for a while.
Note that even if your one plant were a male, this could still be a viable strategy. Males still have enough THC that if I were on this "desert island" I wouldn't just give up on one.
If you're going with a hermie plant, then ideally you'd also want it to be a "mutt" F1 so that it carried a variety of genetic traits. A reasonable seed choice for that might have had parents that look something like this (Landrace Sativa 1 x pure Indica 1) mom x (Ruderalis/indica hybrid 1 x landrace Sativa 2) dad.
This way, with the appropriate selective breeding on your part, you could generate landrace like Sativa plants that might acclimate themselves well to your island, autoflower plants (of varying phenotypes) and even indica-heavy phenotypes. It would be a lot of work to maintain these lines outdoors, but if you were careful about selection, hand pollenization, kept your seeds separate and kept good notes, it could be done. If you're by yourself on an island, you'd have the time, I guess.
Alternatively, if you simply spread the seeds around, they'd open pollinate and after a few generations, only the ones most acclimated to your particular environment would survive natural selection, so you'd end up with some sort of "landrace". You'd do well to make sure you applied artificial selection to maintain potency here, otherwise it might erode.
Having nice tall Sativa phenotypes could be good just to generate hemp-fiber (to make rope and such). Not only are the hemp seeds themselves pretty nutritious, but they also work well as fish bait, another plus.