Try a wider hole, using platforms to span it, and stand on the platforms.
I think you are getting spawns, nearer to the bottom of the ocean, but they can't 'see' you to aggro them toward you.
I did a test of a similar setup, with battle potions, hunter potions (to see enemies), a water candle, and a coincidental blood moon. I could see the jellyfish and sharks near the bottom, but they were ignoring me. When I made the hole bigger (and placed platforms to stand on), they headed right for me.