Marmaris lies on the south-west coast of Turkey, and of the airports in the region, Dalaman (DLM) is the one nearest to it by road. Bodrum-Milas and Antalya both serve their own holiday coasts and are considerably further away; flying into either and driving to Marmaris would add hours to your arrival day. We do run transfers from those airports too, but if you have a choice of flights, Dalaman makes the most sense for this part of the coast.
From Dalaman the journey is a little over an hour and a half to the town itself, which is very manageable even after an early start from home.