Marmara Islands are administerively a part of Balıkesir 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 Saturdays).
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, although Paşalimanı and Ekinlik is served less frequently than Marmara and Avşa, at which practically all ferries call. You’ll have no problems on inter-island transportation once you get to the harbour of any given island.
All of the islands had significant local Greek populations up to 1920s, so some architectural heritage, such as historical houses or monasteries can be seen on the islands, if sometimes partially ruined.
Islands’ telephone code is 266 (+90 266 when calling from out of Turkey).