Agriculture also seems unlikely unless youre a sentient carp or something

We already have sentient carp. Or at least, sentient denizens of the deep.

We already have that here.

Sperm whales communicate with each other in a language as complex and information-dense as our own. It is reasonable to assume they are sentient. We are not discussing the hypothetical sentience of matrix math on silicon chips here. We're talking about another biological entity, with a brain of the same fundamental nature as our own, with a brain much larger than our own, of common evolutionary heritage to our own. And by what other objective measure could we use to measure the sentience of another biological mind, than the complexity of its communication? You could look at behavior. But they're complex social mammals just like we are.

If we are sentient, so are they. I'm sure they have thoughts and as complex of inner lives as we do.

I wonder what their culture is like. Do they tell stories of great hero whales of the ancient past to their children?

Or are they more learned? Are they down there debating the finer points of philosophy like Plato and Aristotle? They couldn't do anything experimental. Perhaps they would become masters of classical philosophy.

Perhaps, more ominously, if we can learn to talk to them...what will they want to tell us? I wonder if they still have a cultural memory of the whaling days. Are there sperm whales alive today who hear tales from their elders about the time humans ruthlessly hunted their kind? Do they doubt they are real, wondering if they're merely old superstitions? These seem so different from the humans you know. (Mostly.)

Or would their culture be so different, that none of these concepts would even remotely map to the things cultural touch stones I cite?