First of all, it's good in a way that you're not hoping to dive at the Similans in August since the park is closed until November. If you want to dive, you can still do some daytrips out of Phuket or from the islands in the Gulf of Thailand.
When you do return in November, Bowmouth has given you the two major options for diving at the islands.
1) You can do daytrips, usually by speedboat and usually consisting of two dives per day. These are not particularly cheap when we calculate the cost per-dive since last season they tended to run about THB 5500 (~$170) per trip to sites at the Similans and THB 6000 (~$185) for a trip to Richelieu Rock. This option is good for people who don't have the time or inclination to spend days on a liveaboard. Also it might be good if a person's budget won't allow enough money even for a short liveaboard since in terms of total outlay it is less expensive to do a single daytrip than to do a multi-day liveaboard trip. But the cost per dive is quite high.
2) You can do a liveaboard. The most typical trip length is 4 days/4 nights. Because these trips generally allow for four dives a day, they can become very cost-effective on a per-dive basis. The high end boats run from about $300/day and up. There are more moderately-priced boats, though, costing in the range of $250/day, and the budget boats for about $160 to $200/day. Understand that even within these general price categories, there is a range of prices and cabin types available. Obviously, if you get on a boat with lots of divers sleeping in dorm-style quad cabins and having access to shared heads, the cost is lower than if you book a boat with two people per cabin, private heads and fewer divers on board.
As far as dive buddies go, tour leaders on the boats do try to pair lone divers with others of similar abilities and experience, but of course any time you get an "insta-buddy" it's the luck of the draw. If you do not want to risk it, you can always hire a private dive master to go along with you. Then it's just you and the DM and nobody else.
In RE kannjihyun's suggestion to stay on one of the islands and dive, this is possible if you book an overnight dive trip starting from the mainland. These trips usually use speedboats to take you out to the islands where you dive the first day, spend one night on the island and dive on the second day, returning to the mainland in the afternoon. There aren't any dive operators on the islands, so you can't just rent a tank and a longtail boat and go diving, but you can snorkel. The national park has bungalow and tent accommodations for rent, food service, a small retail shop, restrooms, and nature trails, but there is no specific support for visiting divers.
