To sign island, make you dominant hand into a fist and stick out your pinkie finger. This is the ASL symbol for 'I'. Next, swirl your pinkie in a circle on the back of your other hand (usually in closed or fist form) to represent the island surrounded by the ocean.
Island is a concept that will not be fully grasped until the child is older. But showing your baby pictures and teaching the sign can help prepare your baby for a new experience. Introducing this concept along with beach and ocean will allow them to feel more comfortable in this new setting.
Geographical concepts are difficult to teach. For island, we waited until our babies were old enough to show curiosity about stories set on islands.