i cant help you with the accommodation, but i have stayed in marmaris a few times over recent previous xmas's , the weather is around 15-18 in the daytime and freezing at night, so winter coats are needed for night time, we have been lucky and had a few nice days where we were able to sit on the beach in t shirts, but certainly not warm enough to swim or sun bathe. it can rain quite heavy again we found it only rained at night whilst we were there so we might have been lucky, so much warmer than the uk.
we found most of the old town near the marina open for shops , bars restaurants etc, the bazaar mainly open. but the main long strip of shops ,bars etc are closed in the winter
hope this helps a bit