Island not really a good place once the infection be around the planes will crush in water and the deads be zombies float over the water and the tide will get them to a beach which will be island most likely u need to put cage around to be safe
Boat is a good idea but how long u can stay in ocean and also the motor might get damaged due to dead bodies in the oceans stuck in engine if u need a good place go to a hill something that zombies cant reach (climbing)