1) It has only 1 manta in the photo
2) It at least shows the central area of the ventral (belly) side of the manta, including the pelvic area to determine the individuals sex. Ideally, it shows the entire ventral side from wing tip to wing tip.
3) It is in sharp enough focus to make the individual spots easy to …. spot, and to determine the manta's sex.
4) When possible, it's nice to have a square aspect ratio.
5) When possible, it's nice to not have anything else in the photo like divers or snorkelers or any other pesky photo bombing fish.