The Aegean is a sea full of islands, and Greek religion used two of them in opposite ways.
Delos was held to be Apollo's birthplace, the ground where Leto finally found refuge and bore him and Artemis, and the island was treated as sacred accordingly. Athens conducted purifications of it more than once, and the rule that resulted was extreme: no one was to be born on the island and no one to die there, so the sick and the pregnant were removed to a neighbouring islet. Graves were dug up and moved. The island became a major sanctuary and, later, a commercial free port, so the sacred and the mercantile occupied the same small rock.
Samothrace, in the northern Aegean, held the mysteries of the Great Gods, and it kept them. Initiates were admitted at any time, in two grades, without restriction by status or origin, which was unusual, and they did not disclose what happened; even the names of the gods concerned are given only obliquely in the surviving sources. Sailors sought initiation for protection at sea, which suited a sanctuary on a mountainous island in exposed water.
The great Ionian sanctuaries on the eastern shore, Didyma, Ephesus, Samos, faced the water rather than the interior, and were reached by sea as a matter of course. The Aegean was not an obstacle between Greek communities; it was the road between them.