I have a boat in Phuket so I will chime in here.
Here is a summary of the sailing weather
NE monsoon(Dec-Mar)-usually drier and the monsoonal wind opposes the sea breeze so you will have often strong(15-20 knot) winds overnight and in the morning that die out by 2 in the afternoon. It is then either stagnant or light winds until dusk. As there is less rain during this period the snorkelling is generally better. However, this season has one downside, tourists and lots of them. Most, if not all of the anchorages, beaches and hongs will be inundated with longtails, speed boats, etc. from 1000-1600 it is downright miserable and dangerous at some of the spots at this time as the local Thai skippers have no regard for the safety
and well being of anybody or anything. I am not exagerating!! The one saving grace is that the yacht charter business is not (yet) overdeveloped in Thailand
so you generally do not have a problem finding overnight anchoring
space after the tourist minions leave.
If I go sailing during this time of year, I will make a point to be sailing during the middle of the day to avoid the tourist crowds. After breakfast we sail/motor somewhere and time our arrival between 1500-1600 to avoid the crowds! If you have distance to cover at a good sailing angle, it is best to leave at first light to make use of the wind. The converse is true if you are headed to windward; leave so you will make your anchorage by sunset for minimum wind/sea to beat into
Monsoon switch takes place between March and the middle of May. I do not go sailing at this time of year unless I am on a boat with genset and aircon, it is often hot, humid, and still. My worst days on a boat have been at anchor
here when it is 35C in the middle of the night counting the hours till morning!!
SW monsoon(June-Oct): This is the "wet" season, although it is worth noting that the rain is usually more prevalant towards the end of the season than the beginning and usually occurs in spurts. The days between the rain storms are IMHO the nicest of the year! If you are prepared for the occasional severe
squall(I have seen 50 knot
gusts here) June-Aug is the best time of the year for a sailing trip. The rain is not constant, the wind is generally more reliable(at the very least you will get a 8-10 knot
westerly seabreeze pretty much every afternoon) and most of the places are utterly deserted. You can pull into the best spots(with one exception see below) at any time of the day and be assured of a space and usually solitude.
There are two main items of interest in sailing around Phuket:
1. The hongs(sea caves/hidden lagoons) and interesting scenery of the islands containing them
Unfortunately, the best of these two items are at opposite ends of the cruising area so with a 1 week charter you will either have to choose or cover a
lot of distance.
In the northeast season you can sample a bit of both by first heading north to Phang Nga and do the hongs and then head
south to Krabi, Phi Phi, (Lanta and surrounding if you have time), Koh Racha, and back. Skipping Lanta this can easily be done on a one week charter. In the SW season, simply reverse the circle.
Phi Phi- Phi Phi don is always busy and the anchorage at Ton Sai Bay is deep, crowded and very rough during the day due to boat traffic. If you want to experience Phi Phi I suggest arriving at sunset and leaving at first light to minimize your exposure to the boat traffic. The sights on Phi Phi le, Maya Bay, Hong Pileh and the bay at the south end are some of the most beautiful sights in the area but you must be in and out before 1000 am. To quote a friend of mine about Maya Bay, "It went from The Beach to Titanic in a matter of minutes." I usually only go to Phi Phi as a nightstop on the way to or from points south. If I do, I am up at first light on the way out and we go have breakfast outside Hong Pileh or in Maya Bay before the crowds show up. it is then a manageable trip to my favorite spot after that.
Koh Rok Nok: One of my favorite places on the planet. Two little islands out in the middle of nowhere, excellent snorkeling and with the exception of other yachts and a few adventurous tour boats, nobody else. I try and stay 2 nights here on every trip.
Hope this helps