Me, I'd probably strike out looking for people. There aren't that many uninhabited islands in this world (not of any size, anyway—little one-tree islets in freshwater lakes don't count). I'd travel along the beach so I don't go in circles, and also from some vague calculation that streams run down to the sea and cities are near water and so forth.
After a while, it would probably occur to me that while there are few completely uninhabited islands, there are quite a few uninhabited places, some many miles from civilization. It would also occur to me that I'm half-drowned and not feeling very good and perhaps I shouldn't be trying to hike anywhere.
At this point, I would probably try to find water. I might actually stick pretty close to the wreckage at this point, if there is any. It's visible, after all. As a modern person, I assume that ships don't just disappear and get written off; as an American, I have some vague idea of what the Coast Guard does, and even if I'm nowhere near America, I might assume that someone's coast guard is out there doing search and rescue. If the wreckage isn't visible, I might try to make some sort of large sign in the sand, but I honestly don't think I'd get to that the first day. I'd be in pretty bad shape, after all.
When I get hungry, I would remember (as someone who read Ranger Rick obsessively as a child) that tide pools support life such as crabs and mussels and what-have-you. I dislike seafood and loathe the idea of trying to eat it raw, but it might be somewhat better than eating totally unknown berries. If I'm actually on an uninhabited island, there might be some ground-nesting birds; if it's the right time of year, I might try to steal an egg. This will probably get me injured, since anything like a pelican is going to be (a) not happy with my nest-robbing ways, and (b) big enough to do some damage. Having gotten an egg, I will crack it and belatedly realize that it's fertilized, which means bloody, which means ewww yuck yuck yuck gross. Whether I eat it anyway depends on how long I've been there when I compose this brilliant plan.
I might look around for fruits I recognize, but—you know, here, I have to stop speculating about what I would do, because I actually have some idea what a totally untouched coconut looks like. Someone who doesn't know that might go looking for little brown globes on trees, like in the cartoons, and walk right by the real thing.
I also realize that if I can't improvise some kind of tool, there is no way on Earth I'm getting one of those babies open. So if I find a coconut, the next thing I've got to do is find something that'll make a dent in it.
At some point, it's going to occur to me that I really want a fire, for heat, food, and signalling. Honestly, I don't think I'd have much luck with that.
It's worth noticing that many of my reactions are completely twenty-first century. I assume that someone's looking and that they have planes, for instance. I assume that while this island might be unknown to me, someone knows about it. If your story is set in another time period you're going to have to ditch a lot of that.
Izunya
The ship didn't wreck on the island. The girl just washed up on it.
