Hey there Mike,
I live and work on Phi Phi, so I can give a bit of advice for you. As for accommodations, you can get a decent fan room for about 400 baht and an AC room for 800 a night. Of course, if you want to go luxury then things can get more expensive. Best bet is to do a bit of walking and looking for rooms once you're off the ferry, since many hotels here don't have any sort of internet presence. Always ask to see the room first, so you know what you're going to get. November and December are high season, so rates tend to go up as well.
As for diving, I'm not going to promote my own shop or anything. I'll let you know that there is a price agreement on the island, so no one shop will be cheaper then the other. There is a scuba shop about every 20 feet here as well. My advice when choosing a shop is to ask to see their equipment, ask about their boat (you'll notice a few pretty poor diving boats parked in the port), and ask about their guides/instructors. Another factor would be where they actually dive at. Some shops take the easy route and dive the same closest sites every day so they can get back early. I would recommend an operation that is willing to go anywhere. My personal local favorites are Bida Nok and Hin Bida (sometimes called Phi Phi Shark Point). During that time of year you can also grab a speed boat out to Hin Daeng which is a great dive site with chances of Mantas and Whale Sharks. That trip is much pricier, obviously.
Anyways, hope this helps and hope you have a great trip! Feel free to ask any more questions you have.
I've never stayed on Phi Phi (except for work) so I can't help you with that one. For diving, I'd recommend The Adventure Club - 
