Stan -- First of all, if your yardstick is Caribbean diving, you won't be disappointed even with our "mediocre" dives. Even the dives here that in relative terms we consider only "okay" rather than "fantastic" have more prolific marine life than anything I've seen at the six different Caribbean destinations I've dived at. Having said that, I agree that since you'll be over here, you should try to dive at our very best dive sites.
Next, it may be important for you to prioritize which aspect of your trip is most important in terms of such a long journey--diving every day or family beach time every day. If it's beach time, you may have to compromise on your diving; if it's diving, you may have to compromise on your daily beach time.
For example, as a compromise on daily diving, you could stay in the Khao Lak area north of Phuket and dive every couple of days, all day, rather than half a day every day. The dive sites are simply too far away for half-day trips. Or, as a compromise on daily beach time, you could get on a snorkel + dive liveaboard boat for a few days with your family (no beach but one or two island visits maybe, and some kayaking perhaps) and after your return sit on the beach without diving for the remainder of your trip.
A third option would be to charter a catamaran for your family and plan your time the way you want it--island hopping, beach exploration, diving, kayaking, snorkeling--as much or as little as you like. The national parks that we dive at are made up almost entirely of uninhabited islands (one reason many of the dive sites require long boat rides), and sailing around to and among them on your private charter is a real thrill, a trip of a lifetime.
My point is that every dive destination has its unique characteristics, and not all of them allow for twenty-minute trips to the dive sites in a little boat. You might find something like what you have in mind at Phi Phi, and there are a few very nice dive sites there, but for the most part the dive sites around Phi Phi are fairly average in comparison with those at the Similans, and again, if you are coming all the way over here, why not dive at the best sites?
