Well, I don't really do true "instabuddies", because I travel with my husband, but I've done some dives with people I absolutely didn't know, and sometimes didn't know much of anything about.
One, last June, was a new diver on about his 7th dive. Bob asked me to dive with him on a "Big Buddy" day. He was a very pleasant fellow and very earnest about his diving. We did a simple, shallow tour and had a good time. He has since become a close friend and one of my favorite dive buddies. Not only is he fun to dive with, but he has a WICKED sense of humor.
Kidspot was another great surprise. I only knew him from Scubaboard, and we got in the water together for our first dive, and it felt like diving with somebody I'd known for years.
And then there's HBDiveGirl, who, with KMD, made up my Monterey team. Again, like beloved old jeans, immediately comfortable.
And the last one is our own Do it Easy, who also felt like an old friend in the water.
I've been fabulously lucky with doing this sort of thing.