We went to Marmaris a couple of years ago at this time. The weather was okay, certainly better than back home in the UK, but not so hot that lying on the beach was a real option.
Not everywhere was open, however. Lots of restaurants and hotels were finishing off maintenance work on pools and barbecue areas etc, (tiling, grouting, painting and such).
There was also a major refitting of the beach going on. Sand was shipped in from elsewhere in lorries and dumped on the beach. There was a pile on the beachfront owned by every restaurant, hotel etc and an enterprising chap with a JCB went from business to business smoothing it out and making the sand presentable. Presumably it's washed or blown away in the close season and they replace it at the last minute.
While it wasn't wildly hot and it rained a couple of days, we enjoyed out week. There weren't that many other tourists about, but the Turks were very friendly and accommodating.
Depending on what type of break you're after, you should enjoy it.