You can totally squeeze in a dive at the dive park, maybe even a couple dives. Call Catalina Divers Supply ahead of time to reserve a tank, weights, and whatever else you need. If you like, book a guided dive with them while you're at it. Last I checked they offered them at 8, 10, 12, and 2, but they might have changed, or they might be willing to work with your schedule. But it's not the kind of place where you really need a guide if you have decent skills and have been in cool water before. If your ferry arrives at 10 and you take a cab or walk briskly toward the round casino building (just over half a mile), get your stuff and gear up, you could be in the water by 11 or 11:30. When you're done diving, there are showers (hopefully they're open again by now; CDS can probably tell you) for $3, so you can get yourself looking meeting-pretty in plenty of time. Maybe pack a lunch for your surface interval if you want to squeeze in a couple dives.
