It might be either the scallop or squid. Both have fast and erratic moves but the squid is worse. The tell-tale sign of a squid is that when it first moves away from you it leaves something faint like a shadow. To catch any of them, there are some steps, as below.
Swim very fast above the creature and try diving down just above them. With luck you might even touch it while doing so.
Dive between the creature and the barrier.
Try to dive in a direction where you are going to swim with the flow of the ocean. Swimming against the tide makes you slow.
Once you have dived, keep your finger on the round directional button and be ready to change direct very quickly. (The squid is notorious for erratic directional changes) In the meantime, press the A button very fast to enable fast swimming.