Where did the diver you saw enter and exit from?
I went there 2 times last week and checked out the site from the observation area of the Sheraton but the waves were smashing up against the shoreline/hotel so we didn’t go in since none of us had experienced the divesite before. The swim from the boat ramp is too far of an effort! I would NOT try it. The Manta night boat dive is around $120-145 for a 2 tanker. Seems to be a no-brainer to just go off shore but that’ll be a mistake if you don’t know the currents and the terrain on how to get to the manta’s.
Went to the Old Kona Airport area and got 2 Mantas to buzz us for 10 mins at 40-60ft depth so as the other divers have said, there’s easier access dive sites to use to get to Mantas at night.