Neknit
I have been on about ten or so gulets, Mostly between Marmaris and Fethiye and the only choppy weather we ever seem to get is when they have an early morning start to get to the Dalyan area if people have requested the trip. Even then I have never felt ill and have only known one person in (approx) 150 passengers to suffer badly from seasickness.
Nothing in the world compares to the chill out on a gulet although you can still be pretty active if you choose.
There is swimming, snorkling, canoe, windsurfer (sometimes), walking (best while ashore) and falling over drunk. You might also get banana boat, jet ski or water skiing.
As an active person I always insist on stirring my own tea.