You don't specify where this facility would be, but mention Sarigerme thermal water, so I'm guessing it will be located there?
Personally I feel that Sarigerme is one of the few hidden treasures left and should be left as it is. There are big hotels there but they've been made sympathetically to the surroundings and are unobtrusive.
The little town has just enough facilities without being tacky.
No rip off prices, reminds me of Olu Deniz 20 years ago, outstanding natural beauty, limited menu from a wooden shack. It's lovely.it still feels like Turkey, not many places do now.
I'm not a fan of modern facilities in unspoilt areas.
I know you mention Jiva resort... Sorry but I think it's ugly and spoils that end of the beach, we all know what happens here, open one lighting shop and 10 more will appear next door, sadly that will likely happen down in Koca Calis too, it will be full of all inclusive 5 star hotels and many restaurants and hotels will end up closing as a result. The resort many fell in love with will be totally different.
The Therme Maris Hotel in Dalaman offers thermal pool facilities and only a handful of people from here have visited, myself included. I certainly wouldn't want a yearly membership.
Dalyan is lovely for the thermal pool and mud bath experience and is lovely to take family and friends to visit on holiday, but I don't think many would go regularly for the health benefits if they had yearly membership.
We have tennis courts locally to us, and rarely see anyone other than Turkish children and adults using the facility. The ex pat population locally here tend to be a little older and fitness activities don't tend to be over subscribed.
I hope my input is useful. Obviously my opinions don't represent everyone, I tend to still live in the old days, in the Fethiye area I fell in love with many years ago and don't necessarily like all of the changes taking place locally. Many people love the changes.
Hopefully others will give you their feelings too.
I think coming onto a big forum to get ideas before starting such a big project was very sensible. Many businesses come and go because they don't do their research.