Hi there,
If it were me, I'd do a liveaboard to the Similan and Surin Islands and Richelieu Rock. The downside is that you'll not have much time on the beach (maybe you'll stop off on one of the Similans for awhile during a surface interval, but otherwise you'll probably be on the boat the whole time); the upside is that you can do 3 or 4 dives per day on many truly excellent dive sites, and whether you depart from Phuket or from Khao Lak you'll not be too far from the Phuket airport for easy access to and from Bangkok. Koh Tao is a bit more difficult to get to, as the nearest tourist airport is on Koh Samui which is a couple hours away from Koh Tao by ferry.
In choosing among the many liveaboards sailing the Similans, the first issue in reaching your decision will be timing -- if you have to "squeeze in" the diving, then the boat's schedule really must be compatible with yours. They pretty much all go to pretty much the same dive sites, but do they go there on a 3- or 4-day cruise which matches your schedule?
After the schedule, the next issue you'll have to consider in reaching your decision will be budget. In terms of personalized service vs. an impersonal and overcrowded boat, some say that you'll get what you pay for. To some extent that's true. Compare, for example, the expensive Nautica with 12 divers, and the cheap South Siam III, and note that the South Siam III info says "28 divers" but that could mean 56 divers if 28 are staying that night on board and 28 others are leaving by speedboat that day (and no, it's not unheard of to have 45 or 50 divers on a South Siam boat during high season ... which certainly wouldn't happen on the luxury liveaboards. But, South Siam is cheap!).
However, there are some smaller mid-budget liveaboards which might be mid-budget rather than high-budget for reasons of comfort, cabin size, etc., rather than lack of personalized service. For example, the last time I did a Similans liveaboard, it was a mid-budget boat with a maximum of 16 divers and 4 divemasters/instructors (at least 3 on any given dive) -- so it was just fine for personalized service, and the food and the diving were great ... but while my DM was fluent in several Scandinavian languages he was otherwise not very impressive (as a DM/instructor myself -- and as someone who has dived as a paying customer in about a dozen countries around the globe -- I have high but reasonable standards), and the cabins were a bit cramped and they weren't well-ventilated so there was a slight odor of gasoline from the engine room (I won't name the boat because they swore that they'd fix the ventilation problem after that trip, and DMs come and go, ... however, the cabins on that boat are no doubt just as small now as they were then).
Here are a couple of links just to get you started in the selection process (you might google "Thailand liveaboards" or "Phuket liveaboads" or "Similan islands liveaboards", etc., for more links); each of these two links has a schedule/calendar function to tell you which liveaboards are available when you are free to go diving, and each gives you an idea of each boat's price range. When you've narrowed it down to which ones' schedules match your schedule, and which ones best suit your budget, then search for those boats by name here on scubaboard.com and/or post a more specific query here for further info. These links are to booking agencies --and using a booking agency may or may not be better than booking directly with the dive operation itself -- and I'm posting them here only to give you an idea of what's available and when.
Thailand Diving - Phuket Liveaboards - Scuba Diving Similan Islands - PADI Dive Courses
Liveaboard Phuket - Thailand Liveaboards - Similan Guide
Note that in general, there is more diversity of marine life -- especially macro stuff such as nudibranchs, shrimps, etc. -- on the west coast (Phuket, Krabi, Similans, Koh Phi Phi, Koh Lanta, etc.) than there is in the Gulf (Koh Tao, etc.). And often the visibility is better on the west coast than in the Gulf. However, if you go to the best Gulf coast dive sites when visibility is good, you can do some great diving, and you can see some marine life which is not so common on the west coast. I worked as a scuba instructor on Koh Samui for more than a year (mostly diving Koh Tao and Sail Rock), and had many excellent dives with whale sharks, Spanish mackerels, bull sharks, reef sharks, etc. etc. ... but I also had a number of mediocre dives in poor to moderate visibility with not much very interesting to see unless one's a novice diver (and Koh Tao's diving industry is based primarily around certifying new divers, rather than catering to experienced ones). As I said above, it if were me, I'd do a liveaboard to the Similans and Surins and Richelieu Rock.
Whatever you decide, I hope you and your husband have a great time. After your Thailand trip, please post a trip report (with photos, if possible) and tell us all about it.
Cheers,
M
