Beach Resorts on Turkey Coast not too far from Istanbul

100-200 miles from Istanbul?Well there are several that come to mind but...it isn't so much the distance as how you'll travel it.By this I mean that those close-ish to Istanbul mean a rental car or bus journey to get there taking c.4-6 hours or going to a further place by 1 hour flight.
I don't know when you are planning to come here but bear in mind that it now high season and pretty much any coastal place will be busy with both domestic and foreign tourists so'quiet' could be hard to find anywhere.So here are some suggestions which are less commercialised and thus relatively less busy but not necessarily as quiet as you may wish.

From Istanbul take the ferry over to Bandırma and head to Erdek on the Kapıdağ peninsula.This is directly opposite Istanbul and has some nice beaches and scenic rural walks and almost exclusively Turkish guests,thus untouched by foreign package tourists.The ferry takes 2 hours and then another c.20 minutes by local bus or taxi.

Take a bus from Harem on the Asian side up to Ağva or Şile,90 minutes ride away..Both are lovely but weekends are especially busy with Istanbullers.

After that you are looking at longer bus journies,i.e. 4 -6 hours along either the Black Sea coast or down to the northern Aegean coast.

If you are prepared to take a short domestic flight then this opens up a few more possibilities.Let me know if this is an option for you and I'll give you further suggestions.

Go to Enez in Edirne by greek border. Sultanice Village is very beautiful in Enez too. You can cross into Greece too.
Go there, you wont be regretful.
Only local tourists go there so its not a touristic town..

The water cleans itself in Enez. Its one of rare places in the world in terms of that.
Its 3.5 hours bus ride from Istanbul. If its far for u, try Sultankoy in Silivri or Igneada in Edirne or Kilyos, Sile , Agva but last four located in the black sea so it could be cold water depending the time of year...