1

I have no idea how heavily booked gulets are right now, but my rule of thumb with boats is to try to avoid booking a boat you haven't seen with a captain you haven't met. That goes for anything from a Nile Cruiser on down; obviously, big cruise ships are another story.
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It's hard to know if fronting up and bargaining a walk-on rate will be any cheaper, but if you see your cabin, the cleanliness and seaworthiness of the boat, and hopefully get an idea of who else will be on board, you can avoid a lot of pitfalls. If you've ever watched a gulet full of flabby drunks screaming over bad blaring disco as they stagger around the deck pull up to a previously-idyllic spot, and tried to imagine how being stuck with that lot would impact your dream of moonlit evenings spent smelling the pine-scented air wafting down from the picturesque ruins while violins play softly in the background, you'll see the advantage of being as informed as possible before you book.

2

The idea of just doing day trips by boat out of various ports has a lot of merit. I've gone from Kas to Kaleköy and Üçağız, with some nice snorkeling stops and a good lunch, and found it great value. There are good day trips from Fetrhiye and Demre as well. Bodrum also provides interesting options. You can then stay ashore at night, for a lot less than gulet prices.

3

I would DEF try to do a Blue Cruise. I think they are a highlight of Turkey. You get to see the coast, the best way possible, by sea. I did a cruise last year from Demre to Fethiye no problem. I stayed at Sabans Treehouses the night before and booked it through them. It will be hot, but its also hot going by car or bus. So why not be on a boat so you can swim as much as possible and then sleep under the stars. Sounds pretty good to me :) !!! Wish I could go back for more...

4

Would the OP mind reporting back on what they ended up doing?

Cheers.

Jon