On Marina Divers' website, I couldn't find anything about them not going to Similans in October. Please, double check.
The dive sites I was referring to were the Similan Islands. Most of the dive sites are a good 2 hour boat ride on the slow boats but their catamaran is fast enough to go all the way out to the Similans. To this day, I've only heard of only one other dive shop able to do this on a daily basis.
They do the basic everyday dive sites, Shark Point, King Cruiser, Koh Dok Mai, Anemone Reef, the Racha's Yai and Noi, but these are the dive sites everyone goes to. They are nice if they aren't over crowded.
Marina Divers also does a one night deal to Phi Phi Island. This is were they filmed the movie, "The Beach".
Believe it or not, my favorite dive of all of these local sites is Koh Dok Mai. Nice wall dive that can become a drift if the current picks up.
I usually go to Phuket in April or in December. I don't know their boat schedules for October, I'm just going by what their website says at the moment.
I wish I could head out there with you. I'll be going to San Francisco tomorrow with the family. I live in Japan so it's a pretty big trip. Just got back from Malaysia and did some diving so I have to suck it up and go for a dry trip with my wife.
Let us know how the diving was.
Chris