Hugo
I'll go with Boi and Whitehead's suggestions on the boats and throw in a middle class one called the Colona. It's a nice boat and for the diving in the Eastern Andaman Sea, it's very good. I know both tourleader and guides well and they are a blast (as much as germans and brits can get sober!) I also like the Aqua One(when they had John Gallick) and the Philkade. Anggun isn't doing so well now. Depends on your budget and what you want. If you can get on the Indian Andaman trips, jump on it as the diving is hella better than the Myanmar Mergui, although sharks are still rare nowadays as they longline there too.
FYI, Mergui diving involves paying the Myanmar government a $150 tax. Don't know your political sensitivities. Personally I don't go to Myanmar anymore (even for a visa run!
I actually prefer Hin Daeng and Muang over Richelieu Rock because it's less crowded. At richelieu you can have 200 divers mobbing anything worthwhile seeing. Getting a clean shot of that whaleshark is doubly hard.
As for topside, I'd fly to BKK and do the royal palace and start trips to the temples and castles of Ayuthaya (1 hr north) and Damnoen Saduak floating markets(2 hours south but get there early for the best light and on a weekend is much better but more crowded with tourists). Then drive through to Ratchaburi and the Erawan National Park which to me is the most picturesque of all the parks, except maybe Khao Sok because of the bigger lake. You can also take some pics of tigers at the Tiger Temple near Ratchaburi and of course the Bridge over River Kwai.
The best temple action is really in Sukothai. Up near Chiang Mai, there's a temple in the mountains from where little monks train in Muay Thai and ride horses down the hill for food.
If you've never driven on the right side of the car, I suggest you hire a driver, which would go for around $80 plus lodging a day. BKK driving is ...errr... invigorating for the heart. If you want a real kick, take a scooter taxi for a ride during rush hour. The itinerary I suggested from BKK will take 3-4 days and gives you a real sampling of central thailand. If you feel you want serious temple, well there's a flight to Siem Reap via BKK. Angkor Wat is magnificent.
I'd be happy to buy you a drink while you're there but I'll be away in Indonesia by then.
Have fun and if you need any more info, you can PM me before saturday. I'm flying off sunday.
here's a sample of Ayuthaya:
