Hi ssdd--I see you're in Tuscon, a long way from Thailand (my daughter lives in Tucson, so I know just how far it really is!)
I have to say that you have a rather confused collection of misguided information about Thailand as a basis for your post, particularly geography and a sense of how far/near places are to one another.
First of all, if you are a university student as seems likely, you will probably enjoy Koh Tao. It's dominated by a very young set of tourists--mostly backpackers--so that by and large the place is geared towards cheap stays, cheap activities, and lots of bars.
But if you are thinking of Koh Tao because you believe that everything on the Andaman side shuts down after April 30, you are mistaken. Diving in the Andaman Sea does not stop after April 30. We're still diving every day here! However, I believe that if you have only seven days, you don't really have enough time to go both to Koh Tao and to the Andaman side islands since you'll use up a lot of time just in travel back and forth, especially if you want to do it cheaply (i.e., not by air). Therefore, my advice, if you've decided on Koh Tao already for the sake of cheapness, is that you should just stay on the Gulf side of the country for the whole time.
For the benefit of those who have simply stumbled on this thread, though, let me add some information in regard to the OP's statements:
• It is cheaper to dive at Koh Tao than anywhere else in Thailand, and possibly as cheap as anywhere in the world. There are a couple of nice dive sites, too (especially Chumphon Pinnacle and Sail Rock), but I prefer Andaman side diving almost any time of the year, as do most experienced divers. A dive holiday, though, for some people, is as much or more about time spent out of the water than about time spent underwater, so if diving is sort of like "icing on the cake," the Gulf side is great; but if diving is the primary objective of the trip, then the Andaman side offers more options, even during the summer months.
• We dive year round from the islands in the southern part of Thailand's Andaman Sea. Only the large marine National Parks of the Similan and Surin islands close for the season between May 1 and Nov 1. Lots of our islands do slow down during the "green" season, but I like it at this time of year--the islands are less crowded and so are the dive sites!
• There is no reason to avoid Phuket. It's a very large island, and while there are parts that I won't go to without a good reason (particularly the town of Patong) there are many parts of the large island that are at least as pleasant as any other Thailand island, and frequently more convenient for those on a tight schedule (like having only a week here) given that you don't spend a lot of time on ferry transfers.
• May tends to be a rather rainy time on Thailand's west coast. In regard to diving, the biggest issue is sea sickness on the boat trips back and forth to the dive sites. However, it's hard to predict. We had a lot of rain a couple of weeks ago, then none at all for a whole week last week, and today it's raining a good deal here. The Thai Meteo Dept is a good source for forecasts and historical data. FWIW, the rain forecast for the east and west coast for today are pretty similar (30% coverage on east coast and 40% coverage on west coast). Here's a link for the east coast:
Thai Meteorological Department--Southern (East Coast)
