Phi Phi is a nice dive location with many dive operators.
If go with a big outfit, you normally end up in a big group.
If you read up on the Phi Phi sites, you will know where to go. Then hire a local boat, tanks... and do it yourself.
Since the dive sites are quite safe, no strong currents, you will not get washed away. Of course diving without a guide you might miss all the known good spots. Also you need good navigation skills, if not it could a be a long swim back to the boat.
Since I know the sites quite well, I normally hire my own boat and tanks, then follow my own schedule.
When I am in Phi Phi, I normally stay at Phi Phi Cabana closer to the main strip. I am told Travelodge (Holiday Inn, not onb main strip) is also very nice.
http://www.sawadee.com/krabi/index.htmOver the years, Phi Phi on land has become quite dirty.