Managed to find fast one so I can record and upload it on Youtube. I hope this video helps.
Pretty much sneak on the creature without hitting A button and dive on to it when you are near the creature.
Thank you for this! I hated going after fast sea creatures in NL, so I was dreading swimming when it was announced because of this... But this will be super helpful and hopefully make those pesky fast ones less frustrating to catch!
Only fast one I came upon was the Vampire Squid, and yes, it was fast. I'm actively hunting the isopod and clam.
Okay, yeah, just caught the Giant Gigas Clam. That sucker is FAST! I did notice that if you're on to a fast one, they'll kick up the sand on the bottom and swim away as you approach.
Thanks for sharing the video and thanks @Pyoopi for sharing this tip. In New Leaf I used to just mash A and then when I was right over them I'd dive on top of them. Looks like it's a new strat this time.