I live there and this is not the best time of year. The SE monsoon is well set in with occasional violent squalls and periods od heavy rain. The waters are not particularly clear at this time of year and you would be very foolish to visit the West coast
of Phuket and further North as it's much too exposed and there are no safe bolt holes. If you do sail between Langkawi and Phuket, there are a few anchorages, such as the Eastern side of Turatao or Lanta, which offer good shelter. Phang Na and the eastern side of Phuket are fine too but, if you are on a short charter, you could spend a lot of it holed up, waiting for the weather
to improve. The best time is end November-ish to April.
On the subject of Box Jellyfish, there was an article recently, on the webpages of Langkawi Gazette from memory.
Sorry, I meant to add that I have used Sunsail out here in the past and they are fine. I have no other personal experience of bareboat