Game designer tangent: the point of having emergent things in your game is less to create a sense of constant surprise, treating systems as a generator of the unexpected that must by necessity wind down eventually, and more to slowly build a world that is surprising on first contact but consistent and learnable over time. With the above story, I now know how to better sense ocean predators.
The recent undersea adventures happened mostly on this rather pleasant (on the surface) world, one I found on the far side of a randomly chosen black hole last month. It's within striking distance (50k LY) of the Pilgrim Star, a historical site I've been thinking about lately, and I wanted a way station in this general part of the galaxy. But the large oceans made it a great place to play through the Abyss storyline.