You should read Children of Time and then Children of Ruin.

Thy're not stable over time, and so would be unable to maintain a coherent sequence of behaviour or 'thought'. Human brains are the most heavily armoured and environmentally controlled part of our bodies because complex behaviours are incredibly sensitive to environmental interference. Something as totally open and exposed as a bare sludge or ocean currents couldn't maintain sufficient coherence.

Your first paragraph almost perfectly described the science fiction novel Blindsight by Peter Watts.

"Heck, what's stopping us from classifying ocean currents as a sentient lifeform?"

Section 6 of "Why Philosophers Should Care About Computational Complexity": https://www.scottaaronson.com/papers/philos.pdf In the context of the rest of the paper, of course.

Besides what others suggested, "Fiasko" [1] and maybe "The Invincible" [2] from Lem come to mind.

[1] https://en.wikipedia.org/wiki/Fiasco_(novel) [2] https://en.wikipedia.org/wiki/The_Invincible