There are two ways one could build an Eiruv on an island:
- One build walls, Tzuras Hapesachs, etc.
- One relies on the "natural" walls. This hetter applies only if
- The ocean does not bring up sediment.
- One could see the whole island in one shot from one location (which is 15 mil).
If one build real walls, they can be as big as one wants as long there are no farms bigger than a beis-saasaim.
Moreover, why do some poskim still bring up the issue of "The ocean does not bring up sediment"?