I still have to catch a blowfish and a stringfish.
For sea creatures I need the spider crab, firefly squid, and vampire squid. I haven't been trying too hard, though.
The one I've put most effort so far to catch are the golden stag and giraffe stag along with the other beetles. Those were a pain.
The walking stick was my last bug I caught, not because I was procrastinating on bug catching but because it was so hard to see I always scared it away. My last fish was a seasonal one I missed out on and I still need the last deep sea creature that I can finally catch this month.
Believe it or not, the GOLIATH beetle was the one I couldn't seem to catch for bugs... Even though I specifically went to an island, cut down the trees, pulled up all the flowers and weeds, and did everything possible... It wouldn't spawn for me at all. I kept getting the rare beetles. When I asked on the forum, someone said I had bad RNG for the Goliath, but amazing RNG for all the rare ones. It literally took me FOUR nights before I caught it.
The char was the hardest fish for me to catch. It took me, and I kid you not, 110 baits before I finally caught it.
I do not recall having any issue with the deep sea diving. ACNL taught me better... ACNL diving was horrid because the sea creatures would go underneath the nets where you couldn't reach them. At least in ACNH, you can get them in a corner and they can't escape
But I have managed to complete my museum in full :3