They distribute tender tickets beginning at 7 Am for those not booked on a shore excursion. So, I guess it depends how early and where you are in line. We actually had shore excursion tickets and were on the first tender. We had to wait 45 minutes, on the tender, before getting off because the Kona officials said "they didn't know we were coming?" Anyway, I would say you would get there by 8:30 if you got in line early.
BTW, if you are interested in snorkeling, the tour we took was the Captain Zodiak and it was really fun. They take you to the marine sanctuary where the Captain Cook monument is. I must tell you, it is done on a Raft Boat so the boat is bumpy and you have to hold on tight! We really enjoyed it.
Another idea I would recommend is snorkeling at Turtle Beach, with the turtles. There is a bus at the pier that will take you there with equiptment for $20 pp. I almost wish we had done this and saved $160! Have fun!