Off the beach snorkeling means on Krabi side of things (between Phuket and Krabi), all those islands will be fine, as well as small beaches like Ralay. In this day and age, air conditioning is easy to find just about everywhere.
Good dining options means a larger island like Phi Phi and to a lesser degree Ralay, but if you can eat Thai food, finding good food is never a problem in Thailand. Outside of Patong, you don't have to worry about too much night life in July.
One issue with a lot of Thai beaches is that they are long sand beaches with nothing to see in terms of snorkeling. Going up along the cost in pang nga, you will not find anything worth snorkeling. Swimming, yes, but nothing to see in most places.
From Phuket, you will notice a nice little island about 1km from Kata beach. Don't even think about it. As soon as you get within a couple hundred meters of the island you discover that the place is full of jelly fishes and that because jetskis keep crisscrossing the place, they often get caught and shreded all over and your whole skin starts itching. Not my fondest memory.
On Phuket itself, going to the south most beach and crossing over to the small island shouldn't be too bad for snorkeling, but I still would recommend the Krabi area instead. It's also more sheltered too which is a good thing as the rainy season should have started in earnest, causing the waves shaking up the bottom (poor visibility).
If you are looking for places a little out of the way, head to Kao Yai/Kao Noi. For some reason these 2 islands never got popular and I suspect there are some decent places to snorkel around there. At worst, it's 5km from the small Islands (where you would go if you book a snorkeling tour on Phuket) and 10km to Krabi
A lot of people complain that Phuket suffered from mass tourism, but few people hardly ever go to Kao Yai/noi (yai means big, noi means small).
Word of advice, use Agoda.com for your hotel bookings. As soon as you are looking for something more than a hole in the wall, they tend to have the best prices by far.