Its very true to chase them to the edge of your island. I caught the gigas giant clam and the giant isopod this way
you can creep up on them by just using the joystick, no a to go faster, then when you're right over them, dive and they're a cinch to catch. i spent half an hour trying to catch my first flower basket, have caught three more with no issues since then.
Chase all "fast" creatures to the corners of your "island" keeping the creature near the fence line. It takes time but they can be caught quite easily if you do this. I caught the king crab, snow crab and venus flower basket this way.
I'd say this one has even faster speed than "fast". Even if you swim slow towards it it'll swim away with speed and you cannot catch up to it if you chase it.