Hi there,
Thanks Michael. Yes, typically the worst of our monsoon season in the Gulf is late Oct to December. Normally its pretty nice again by xmas/new year time though. We've already had a few storms in the last week, but now its fine and sunny again for the moment. Unfortunately every year, the wet season is getting more difficult to predict - last year it was late and the wind and waves lasted through into Jan/early Feb. And the year before there virtually was no monsoon at all. Whatever the condition though, it's nowhere near bad enough to stop diving - and lets face it, many other SB members in less tropical locations dive in way worse conditions on a regular basis and consider it 'normal', so we're pretty spoiled really! On a bad monsoonal day here, viz can get as low as 5-10m, however for the majority of the year, 12-20m is more like the average, with some days or weeks in excess of 25-30m.
As far as shops being closed on Samui, I'm not sure. On Koh Tao some centres close for the month of November for boat and equipment servicing etc, but many remain open. For example we close from the 1st-7th, but after that its business as usual again. To be honest I dont know the operations side of any of the Samui centres well enough to know what they do. Purely from a professional POV, I would like to think you'd at least get an out of office reply if they were completely closed for the season though. You would think, no?
The others are right in what they say though, most Samui based centres just charge you a heap more money and speedboat you to the Koh Tao dive sites anyway, so it may be more viable just to base yourself here for a few days while you get your dive fix. If it helps I'll attach a link to the weather forecasting system we use, which gives regularly updating forecasts a week in advance. Bear in mind its not fool proof though - the 2 nearest weather stations are on Samui and the mainland respectively (a distance of roughly 80km away), so there is definitely some margin for error!
WindGURU: Thailand - Koh Tao, Mae Haad Bay
Hope this helps, feel free to PM me if you need more up to date info nearer the time