I've met various deepwater fishes, like the shork, octopuss and dancing seaweed. I've befriended the mermaiden and tried out all the options she offers for trade in shells when lured with a piece of bait: oxygen tank, guide to treasure, and fast dive. I've upgraded my scope to max and the other equipment to between level 3 and 5. I've found a key on a skeleton and used it on a chest. I'm regularly gathering so much treasure on a dive that I hit the weight-carrying capacity limit, even when I skip the less valuable stuff. I've made a dive that was frankly deeper than there's any reasonable need to go (fast dive, two O2 tanks, and two or three air pockets), but there didn't seem to be anything more of interest down there.
However, I see that some people are talking about a mermaid's kiss and her giving you extra air (presumably directly, not in a tank), and I don't think I've encountered that.