How to get a swimming costume and go swimming
The first thing you should do when logging into Animal Crossing: New Horizons is to go get yourself a fresh new swimming costume. Gotta make the most of summer.
Once you've gotten a diving suit from Nook's Cranny, you're ready to go swimming in the sea. No option to go diving into the rivers of your island unfortunately, but you will find more than enough to do in the sea.
You can enter pretty much anywhere, but you will only be able to emerge from the sea on beaches.
How to catch sea creatures
Of course the main reason to go swimming, other than, y'know, summer, is to get yourself some sea creatures. You will spot the creatres from the bubbles they'll leave trailing to the surface, then you just have to tap Y to dive.
Swim over their shadow while underwater to catch them, but watch out, some are fast and will run from you, while others stay still.
The sea creatures are a new brand of creature being added to Animal Crossing: New Horizons, and they even come with an update to your critterpedia. There are dozens of sea creatures to catch, though you won't be able to get them all instantly. Like any creature in the critterpedia, some sea creatures will only appear at certain times of day and year.
More for the museum
This one is simple, and I'm sure you already know all about this. Once you've caught your first sea creature, you can donate it to Blathers, and he shall officially start accepting sea creatures. He even has unique trivia for each one, bless him.
The sea creatures will be tucked away in the same tanks as the sea fish that you've been catching this whole time, and they make the tanks look much nicer and more lively.
This is good for me, as I've long since filled up all the spaces for fossils, and I was looking forward to having something to hunt which wasn't Redd's artwork…
Finally, we've got some brand new shells in Animal Crossing: New Horizons. While walking around my island I noticed some blue summer shells and a bunch of other unique items.
You won't be donating all of these to the museum or anything - aside from the Giant scallop from the sea - but you will be able to collect them, and inevitably find some new DIY crafting recipes.
Of course this is in addition to all of the other new things you would usually find in Animal Crossing: New Horizons each and every month. Not bad…