I used to manage the Nautica Divers concession at the JW Marriott some years ago. More recently, Sea-Bees has had the kiosk on the JW Marriott property. They are an excellent and well-organised operation, with first-class day trips on very well-appointed boats, as well as trips to Similan.
However, as noted above, you will be FAR from the action (both in terms of diving, and nightlife). Sea-Bees arranges all transfers to / from Chalong Pier in their quoted prices (Similan is basically closed for diving in June), but it's a long day for just 2 dives (3 on Sundays)--up early, van transfer 45 mins. each way, plus boat and dive time, etc....
If you MUST stay at the JW, then Sea-Bees also arranges "local" diving with a contracted dive operation on Nai Yang Beach (just south from Mai Khao), which is OK, but not great (and even less interesting from June-Sept). The advantages : You will have a more relaxed schedule and can do 1 dive (morning or afternoon) and still have time left over to enjoy the rest of the day.
And for the "out and about" portion of your stay, better count on renting a car..the prices for the "limo" service to Patong / Karon / Kata are very high from JW.
Honestly, if you are really set on doing the most dives possible in a short time frame, I would re-consider your accommodation location.
Have a great trip.