I love Sardinia, and wish we were going this summer! We have been a few times, and stayed in Isola Rossa, which is a small village on the North coast. The Hotel Marinedda is nice, friendly staff, good food, close to a lovely beach, but we have only been outside of the summer holidays when it is quiet.
There are lots of great places to visit, depending on what you like. We flew with Thomson and their website has info on the main attractions.
I particularly enjoyed visiting Il Muto di Gallura, it is an agriturismo. There are loads of them all over Sardinia. Some you can stay at, others just have restaurants but they are basically farms with tourist facilities. We have been to others but that one was the best, with fantastic food.
We have also visited Corsica. It has stunning scenery; beautiful beaches and some quite spectacular mountain passes. For me, Sardinia is better due to the Italian influence.
Car hire isn't cheap, and nor is fuel. Alghero airport has the usual big chain hire companies so just check online for the best price before you go.
I can't comment on the snorkelling, sorry.
Hope this helps.