he doesnt always show up when you catch a scallop, to my experience. ive caught a lot of scallops and he only shows up very rarely in those events. maybe im remembering wrong, but i remember denying giving him a scallop the first time, thinking hed show up if i caught another one, but he didnt show up when i caught another scallop. i think its always been in the game as a chance of showing up.
I've found that sometimes Pascal doesn't appear if you catch your scallop too close to the beach. Sometimes a distance you'd think would be fine doesn't work, as happened to me a few days ago. But generally if you're far enough away from land he should appear every time.
I only dived to complete my Museum, this is the first time in 3 1/2 years that I am doing it just for fun, lol. I remember him showing up the two times I did collect Scallops, but that's about it.
He also won't appear if you don't have a free spot in your pockets (since you'll need the spot for the item he gives you). So either sell or release some of your diving catches when Pascal hunting.