I went to Sardinia last Summer and I am so keen to go again.
The water is water and so so clear. I remember snorkling in 5m deep water and you can see the ocean floor as if it was 30cm away.
We hired a villa with some friends and we were on a cliff with amazing views. Car Hire is a must as the island has so much to offer.
I stayed close to Cagliari abut 30mins drive and it was easy to find. The locals are helpful and friendly. Gelati is cheap and good to boot.
To find a cheap flight to Sardinia use www.whichbudget.com and select where you want to fly from and fly to.