The one potential problem is that in May, the better diving is usually at the sites which face West, which means those shore dives you don't like so much (Hejie, Ding Baisha, Shan Hai, etc.). But on a busy weekend, with enough customers, the boats will go there. And anytime of year, if the weather is calm, the boat diving sites in the bay are beautiful: Da and Xiao Lao Gu, Dulijiao, Blue Hole, even some of the shallower sites like Jin Shui Lou, etc.
But September often means typhoons, or at least bad viz from the previous week's weather, etc. Go in May.
But September often means typhoons, or at least bad viz from the previous week's weather, etc. Go in May.