Low season weather in Phuket ... sometimes great, sometimes OK, sometimes not very nice at all! Very unpredictable, I don't like to even say what it will be doing tomorrow.
If you (dbatts2002) plan to stay in Phi Phi, then that's the best way to dive at Phi Phi.
This question brings up a point which needs to be mentioned about Phuket... Most people don't seem to realise that Phuket is so big. It's an island, but it's a big island, with an area of around 540 square kilometers. From the northern tip at the Sarasin bridge, to the southern end is over 50km. Low season often sees people staying at nice hotels in the Northwest of the island at Mai Khao Beach or Bang Tao Beach, hotels like Marriott or Sala Phuket, Outrigger Laguna or Anantara - they have great low season deals, but .. all local day trips in low season depart from Chalong, right in the south of the island. If you stay at one of the northern beach areas, you can have a 1 hour transfer each way, and substantial extra transfer costs for diving day trips. If you plan to visit Phuket for diving in the low season, or even in high season if you plan for just some day trips, best to stay closer to Chalong. Kata or Karon beaches are closest of the main beaches. There are also hotels in Chalong and Rawai area. Patong is not too far, but even from Patong the transfer can take half an hour.