I love how completely alien they are compared to us. The sort of decentralized nature of their nervous system compared to our path of evolution. (I'm no marine/evolutionary biologist so I'm probably butchering the terminology...) I loved reading the two pieces Sy Montgomery wrote that are in Orion Magazine. https://orionmagazine.org/article/deep-intellect/ And the follow-up: https://orionmagazine.org/2011/11/intervie…
If you want to read a book which takes this idea to the extreme check out Solaris by Stanislaw Lem. It deals with a living ocean which has been completely incomprehensible to humans for hundreds of years - a neat thought exercise on the limits of knowledge.