Fyi, one of my coworkers tried to night snorkel at Mauna Kea a few months ago and were told off by the valet
"...showed up at the resort for an evening drink, and the resort gave us a hard time about trying to snorkel on our own at night out there. Basically the valet parking guys noticed me trying to smuggle out my not-very-subtle bag of snorkeling gear and was like, "is that snorkel gear? You can't do that." Maybe we could have gotten away with it if my bag had been more reasonably sized, but I know they're doing official snorkel tours starting at the resort now so they may also be cracking down on it."
so YMMV