aloha,
i live in captain cook on the west side of the big island. i am familiar with the manta site you have asked about. It is a difficult dive with entry at the nearby harbor and a surface swim of over 100yds.
unless you are an expert diver i wouldnt recommend it.
also the folks at the hotel really discourage diving at night with the mantas.
they are afraid of behaviour modification of the mantas presence there and because it is a night dive and the currents are hazardous here they would be the ones to call for rescue.
that being said, it is a doable dive. i wouldnt do it because there are so many other easy places to dive from shore.
if you are in the are and need a buddy for shore diving let me know. i live within 5 miles of some of the best shore divin in the state.
reply if you are interested in more info or need a buddy.