I got this usefull info below off a local website. Obviously conditions can vary widely due to weather. Let me know when you are planning to drive down. I may know of a group heading to the island that weekend and I can steer you in the right direction.
Rich
Catalina Island Underwater Visibility Visibility is normally 40ft. The best underwater visibility at Catalina Island is from July to December, with visibility peaking from September to November, 50-60ft visibility. Black sea bass and large baitballs visit in the summer and fall. April - June can have occasional plankton blooms that lower vis to 20ft.
Catalina Island Water Temps The warmest temperatures are from mid-July to late September, almost 70 degrees near the surface, and the coldest waters are from March to early June, 56-59 degrees. The dive boats can be crowded in July and August. Jan to March can be clear and cold, and April - June can have occasional plankton blooms. Jan to June there are thermoclines as you get deeper (60-80ft), below the thermocline the water will be cold (low 50’s) and clear (70-100ft visibility).