In May, I returned to Club Ocellaris for the 7th time, so obviously I'd highly recommend them. The dive guides are the best I've ever encountered (Peri and Alexis, in particular), the food is delicious, the staff is great and the accomodations are fairly basic but comfortable.
I've always visited in April or May except for one trip in September 2006. As you're aware, it is the rainy season. The biggest effect it had on the diving was that the seas were larger and choppier than in April or May thus limiting the viz and the ability to dive some of the exposed sites. Fortunately, most of the sites were still accessible throughout the trip and we even saw a whale shark (something I've never seen before or since in Anilao). Also, the water was a few degrees colder, but that was no big deal.
On choppier days when the boat trip would have been particularly uncomfortable, they hired a jeepney to drive us and our gear to the given dive site.
Overall, the number of critters we saw was slightly less than we typically find in April or May but not significantly.