I've brought my own snorkel gear to Coco several times and have always seen lots of fish near the fake ship werk and more prominently at the wall of cannons as well as just out in the open water, I've seen rays every single time as well and a shark at least twice. Only once (Sept. Of 23) did I ever see jellyfish, and they didn't seem to care about me.
If you've never been, the best place to start is from chill Island Beach where they have the little lagoon/coves. You can swim out to the floating docks and that'll get you close to the cannon wall, the ship is a bit more of a swim ((and from what I've gathered that airplane is no longer accessible))
Red is the best floating dock to hang out at, yellow is where the cannon wall is. I didn't circle the fake ship since I was unsure where it was on the map but it further offshore from the red circle, heading toward the top right of my screen shot but not thaattttt far
Edited to add a photo I took while there in February this year.