Okay so here's where I was confused. Theres actually 4 tentacruel. The 1st one in the cave on decention down you ignore. Once in the next little area, turn around to see the Male you need for the quest. Hit it with an illumina and hit the melody really quick. It should move down the path where you'll see it at the end

>>1 When you hit him with the orb, play the music right after. That will make him move down

Need to hit the male with an orb and play the melody until you see him heading downwards

>>2 Thank you for providing something for the New Pokemon Snap Walkthrough Team to look into! The walkthrough has been updated with an extra step. Thank you for helping improve our site's walkthroughs and for supporting Game8! ♥

When you are descensing, there's 3 Tentacruel. The male that is to the lower right needs to be hit with an orb. Don't hit the female or she will block him from joining the Mantine. This should put him at the altar.

This is not correct, so far. There's something more that needs to be done, because all I have from this is a lone Frillish looking sad, in needs of a partner.