My buddy had a PADI Scuba Diver card that he got in a YMCA course in the late 70's. We started diving together in 2008 and did about 40 dives together over 4 years and then went to Kona for a week. He forgot his card and Jacks Diving Locker wasn't going to let him dive. They tried looking him up by his name and couldn't find anything. My buddy called his son at home to find his card and check for any info. They couldn't look anything up as those cards had no certification numbers. They wound up saying he could dive but I would have to be responsible for him.
Jump ahead 4 more years and he's still using the Scuba Diver card until we dive with Dressel Divers in Mexico. I had convinced him to get AOW certification at this time since it was "free." The person didn't recognize his card, made some calls, wouldn't let him do the AOW until he first got OW, which they let him do free, except for the cost of materials. Glad we were there for 2 weeks.
Regarding the Big Island, yes, do the Manta Dive, although I did it as a snorkel. It was cheaper and we had non-divers with us. I wondered how the divers liked kneeling on the bottom for 45 minutes. I think it's a matter of if you want to see the mantas swimming up or down. On the surface they would barrel roll and brushed up against us several times. Got some nice pictures.
I'll be on the Big Island again in the fall for a couple of weeks and besides the Manta dive/snorkel, plan to also do the Magic Pelagic (Jack's Diving Locker) dive, also called the Pelagic Blackwater dive or just Blackwater night dive with other ops.
As to OW/AOW and depth of dives, on that first trip to Kona, I was only OW at the time (and my buddy Scuba Diver) but 4 of the 6 dives we did were below 60 ft. The other two were 58' and 60'. My buddy, who had a computer was allowed to dive his plan, while I was supposed to surface sooner because I was still analog - and remember, I was supposed to be responsible for him!