I don't know the halacha l'ma'aseh on this topic, but the gemara definitely discusses certain natural boundaries which constitute an eruv (see Eruvin 8(a) and 22(a)-(b)).
In order for an area to be a Resus Hayachid (RY) it need to be compleatly suronded by mechitzos (halachic walls). the chances of this island having naturally occuring continuos natural boundries are so small that unless you go map them out, there's no reason to discuss it. but even if there were mechitzos, you'd still need to deal with sechiras reshus...
Basically, unless a mumcha in practical eruvin approves it, don't make assumtions that theres anything there....