I live near Fethiye in southern Turkey. The only ferries that run in winter do so from Rhodes to Marmaris and run several times each week but I doubt you will get accurate information until you arrive in Rhodes. This year the hydrofoil operator from Fethiye to Rhodes has bought a new vessel and is currently running crossings four days each week. He may continue during the winter (as he has expense of new hydrofoil to meet) but has not yet announced any definite plans. Finally there are daily crossings from Kas (further east on coast of Turkey) to Meis a tiny island which has ferry connections to Rhodes. Also from Kos to Bodrum and Kos can easily be reached from Rhodes. Both the Kas - Meis and Kos - Bodrum boats continue all winter. My advice is not to worry about this now but, as soon as you actually arrive in Rhodes, get down to the harbour and you will make it happen.