unfortunately I think it's just luck I managed to catch 2 in a row (the only rare diving creature I've caught so far) like 30 mins ago (at around 2pm est? maybe earlier? can't remember exact time lol), I just went for every creature I saw and eventually I saw the like fast moving bubbles... it's also raining in my town, not sure if that helps increase spawn rates for rare diving creatures like it increases rare spawns for fish.
You just have to keep searching for them. I find maybe 1 rare creature (Squid or Isopod) every 2 hours. You just have to keep looking and hope you get lucky.
I swam into one while I was half-asleep. I'm incredibly jumpy which is why I don't have luck with bugs. It helped that I was relaxed. I read somewhere on here that you should sneak up without pressing A, so I did that.
Only worked one time so who knows if I can manage more.
Them showing up is random. If you want to catch them, just swim towards them without tapping or holding the A button. When you get close you can dive on top of them. Pyoopi gave that tip to someone who then showed people on here. I hope it helps.