Best time was 20-30 years ago. You are way too late.

I haven't been to Phi Phi since summer 2004 (pre-Tsunami), and even in the so-called "low season" it was overrun and a mess. Perfect case study in how greed makes for environmental disaster. From the tourist point of view, bad quality accommodation for the price, and poor food as well. Natives that aren't friendly and just don't give a d@mn. Maya Bay on Little Phi Phi was OK for about an hour early in the morning before a fleet of noisy, oil-belching longtails arrived with crowds...as long as you were OK with hiring a private boat. I've heard it's gotten much worse now, and not only from this article. The Thai tourists are much worse than the foreigners for irresponsibility and trashing the place.

I suggest that if you absolutely must go to satisfy your curiosity, go out of Phuket as a daytrip and don't go in high season, even if it is cooler and the water calmer. You'll be so-o-o-rry...... May I suggest that in lieu of Phi Phi, you book a (preferably overnight) sea kayaking trip in Phangnga Bay with a responsible operator like Sea Canoe, or John Gray Sea Canoe, companies--google and ye shall find.