I dug underground from the cactus island under the water, and filled it with TNT, and ignited it. Turns out there was a cavern deeper under that. That opened the ocean to this cavern, and oddly, it created a giant waterfall inside the cavern, but didn't affect the ocean surface. So now we have large pool sized entrance you can swim up and down to get into a cavern.
I think it should be narrowed, right now it's about five wide, probably could be three wide instead to look better underground. Or we can layer glass blocks and turn off the water fall, instead having a glass ceiling with water on top of it.
Can't sleep, clown will eat me.