I did tell about it in my last post to Kyuu explaining it. The tentacles are formed from a basically unlimited supply of sea water so there rooted all through the island floor. They detect movement, and nearby chakra signals. These signals then basically 'ripple' back through the tendrils. The tendrils are connected and controlled by Katsuo via an artificial nervous system extending off of his own. With his enhanced senses, he's able to track the point of ripple origin and detect the amount of bodies causing the disturbance. If he's come across the person before he'll likely be able to recognize them by there unique chakra signature.
Think electric impulses, but with a water stream and chakra bursts. Don't take this as a random broken ability Shen, my physics work out. I just took advantage of Naruto's foggy chakra limitations.
Frankly, just about anyone could do it. The only real skill needed other than similar jutsu is intense concentration to maintain the complex form of the tendrils and bubble simultaneously.
I'm honestly feeling like you've got a problem with most abilities that damper your plans. If you've got a problem with my move, please address it rather than assume I'm trying to run the whole rp how I see fit.