The word Monsoon simply refers to a reversal of prevaling winds.. it is not always a bad thing. I have had many happy weeks at my villa in Phuket in the middle of summer without a drop of rain..
I would feel 1 MILLION times safer in Phi Phi than I would in Bali. No one is going to try and blow you up on Phi Phi, there is little if not NO unhappy Muslim influence. Everybody there is there to Chillax, smoke some pot on the beach and enjoy themselves.
FOR WORLD CLASS AWESOME DIVING>
Go to Sipidan on Borneo. It's centrally located, SUPER cheap, and the best , clearest water anywhere. Also, well clear of usual storm tracks.