I did a day trip and then the dusk/night trip with Jacks Diving Locker. Pretty nice facility, would consider working for them. The price was the highest but by the time of the manta dive the crew was comfortable with me. Let us go in early (w/day trip buddy) for a 7 minute semi-private show.
There was a pretty strong swell, so when the others finally made it, they got tossed and tumbled quite a bit. The mantas kept up their feeding passes, even as 4 divers surged back and forth totally out of control, one of the best shows I have ever photographed (tightly gripping boulders with forarms and knees).
Filled up my card, backed off to review and deleate about 7, then quickly fired them. It's shallow so after 40 minutes we moved out to deeper water, with most following the guide for a night dive. I chose to cut my dive short and surface in the manta haze.
If it's calm you might stay longer but it was hard work in strong surge and the night dive part is anticlimatic, unless no mantas show up.
That's not much of an answer but I couldn't just leave you hanging.