Marsh Island is a South Pacific island inhabited by Deep Ones, and was named after Captain Obed Marsh of Innsmouth, Massachusetts who discovered it in 1830. The island also happens to be located near Zara's Kingdom.
British Captain Corcoran visited Marsh Island and encountered a small group of Deep Ones in which he taught them to learn how to sing.
Marsh Island is based on the island from H.P. Lovecraft's The Shadow Over Innsmouth (1936). In the story, Obed Marsh, travelling through the South Seas, discovered an island on which lived a "woman" he took as his bride and who returned with him to Innsmouth. The "woman" was actually a "Deep One," a member of the aquatic fish-humanoids who worship the evil alien god Dagon.