Well you have more boat choice from Phuket, fewer from PhiPhi and AoNang and even fewer from Ko Lanta. The more local sites I like are Shark Point for coral, leopard sharks and critters (At most C on the global scale) and King Cruiser (C on a global wreck scale). Ko Doc Mai can have a few nice surprises in its walls but bad viz and current can hit it often. Racha Noi is the "pelagic" dive with occasional Mantas etc. Otherwise it's a rumble dive where you look for critters in them.
Off Phi Phi, BidaNai is decent (D+) and Loh Samah Bay (C+) is probably the best critter dive in the area, with nice nudies and crustaceans in the area. Quite nice on night dives.
What I suggest is you do a Phuket 2 day trip with an overnight in Phi Phi. Calypso Divers uses MV Greta (which is a quaint lil boat) to hit the 7 best sites for 7 dives and an overnight in Phi Phi.
Further south of Phuket you have Ko Ha (D), Ko Rok(D) and Hin Deang/Muang (C to B- depending on viz and fishlife). The Hins are the best sites for soft coral (purple like the name) and life in Southern Andaman, but not big on fishlife (as are most places in the Eastern Andaman Sea.
You can access the Hins from PhiPhi (speed boat), Ko Lanta (loonnngggg boat ride) or Ko Mook/Trang (1 hr if I remember properly). Trang is nice as it's a national park and undeveloped compared to the busy Phuket/Krabi area. Then there are dugongs in Ko Libong (but no swimming with them without permits... and that's IF they let you near them!).
Teow Hai Sanook!