The best approach for success at the pirate cove is getting yourself properly equipped first, so grab an iridium rod and either a trap or cork bobber - these will make a huge difference. You might want to bring some seafoam pudding too since the extra fishing skill really helps with those tricky catches.

Something interesting about fishing here, you'll want to position yourself on that eastern plank near the cave entrance and cast about 5 tiles out from shore. Don't worry too much about the water looking darker or lighter, that's not as important as the actual distance. The stingray is probably what you're after since it's unique to this spot, and while it can be tough to catch at first you'll get the hang of it with some practice.

When you're actually fishing, try to use quick taps instead of holding down the button, it gives you way better control over the green bar. Those blue sparkly spots in the water are your friends too - fish bite way faster there and you're less likely to pull up junk. If you're having trouble with timing, maybe practice on some easier fish in the mountain lake first just to get comfortable with the controls. The fishing zones here are pretty demanding but totally worth it especially when you start getting those stingrays, which by the way make amazing fish pond additions if you're interested in dragon teeth or battery packs. Just remember if you're going for the midnight squid, they only show up from midnight to 4 in the afternoon on sunny days so plan accordingly and bring some coffee to keep your energy up during those long sessions.