Marmara Islands (Turkish: Marmara Adaları) are an archipelago in the Sea of Marmara (Turkey), located in its mid-west portion, near its exit to Aegean Sea (Dardanelles).
Marmara Islands are administerively a part of Balikesir Province.
Turkish is the native language spoken in the islands.
Year-round boats which can accommodate cars from Tekirdağ (on the northern/European coast of the Sea of Marmara; costs 15 TL/person) and Erdek (on the southern/Asian coast of the Sea of Marmara). There are also pessenger-only fast ferries  from Istanbul during summer months, which cost 40 TL/person (or 30 TL/person with a slower ferry departing on Satrdays).
Most ferries which connect mainland to the islands call at almost all inhabited islands, so they can also be used for hopping island to island. You’ll have no problems on inter-island transportation once you get to the harbour of any given island.
Islands’ telephone code is 266 (+90 266 when calling from out of Turkey).