Hi,
I was just in Sardinia last summer. The water is exquisite. I highly recommend you hire a private boat for a day long tour to explore the Arcipelago della Maddalena, tiny islands with waters clearer than a swimming pool. Phenomenal.

I believe there are 3 Starwood hotels these days, Pitrizza, Cala di Volpe, and Romazzino. All phenomenal, and can be worth the 800+ Euro cost if you understand what it's all about AND are interested in that kind of thing.

The prices are that high because these hotels are like a private club, with the same families going every year in the summer, the families who also go to the same elite ski resorts in the winter.
It is a very exclusive atmosphere, and for some people the price is worth it and for others it is not worth it at all.

If you choose not to stay at one of these hotels, you can find a nice 3-4 star hotel slightly to the south. I can recommend Li Cuncheddi as a solid 3-4 star hotel with fantastic beach and swimming. The bathrooms are a little odd, and the food in the restaurant is not exceptional, but I enjoyed staying here for a few days.

But to really experience the unbelievable clarity of the Sardinian waters, make sure you spend at least one full day at the Arcipelago.

Have a wonderful time!