James Cambias' A Darkling Sea is an outstanding hard SF novel that does very, very well what is very, very hard to to: convincingly portray an utterly alien species. It was a challenging idea, creating an intelligent species that lives underwater around hydrothermal vents. But he pulls it off. Here's the basic setup of the book:
We begin on a remote scientific outpost stationed on Ilmatar, a watery moon very much like the moon Europa in our solar system — and it's in orbit around a gas giant very much like Jupiter. Protected from cosmic radiation by kilometers of thick ice, a vast civilization has evolved in the deep oceans of the moon, its cities rising around deep sea vents that provide warmth and nutrients. It's a fascinating idea, based on what we already know about extremophile creatures that inhabit these volcanic vents on Earth. Just as they do on our planet, vents on Ilmatar release nutrients that support many plants and animals.
Our human explorers know very little about the moon's intelligent Ilmatarans, crustacean-like creatures with a kind of early Victorian culture where science is still fairly crude and there's a rigid class hierarchy ruled by landowners. We get a lot of crunchy, hard sci-fi details about the scientists' underwater facility, and their nanotech diving equipment that converts the local seawater to breathable gas. Cambias also lavishes incredible detail on the million-year-old Ilmataran culture, treating us to images of their ancient vent cities, showing us how they write books by tying knots in string the way the Inca did, and even exploring how, blind in their dark undersea world, they "see" only by sonar.
We meet Broadtail, a young Ilmataran scientist who studies his culture's own ancient history — and, later, human culture too. His human counterpart is Rob, a tech expert on the human expedition to observe the Ilmataran culture and environment. Rob is involved in an unfortunate incident that brings the two alien cultures face to face: He's filming one of the human scientists, a kind of Jacques Cousteau-style celebrity, when things go wrong. The celebrity swims too close to Broadtail and his colleagues, who perceive him with their sonar senses for the first time — and collect him as a scientific specimen so they can dissect him in their lab.
This proves to be a grave diplomatic mistake. Not only are the Ilmatarans now aware of the humans, but the humans are in violation of a non-interference treaty they've agreed to with the scientifically-advanced but isolationist aliens known as the Sholen. The bulk of the novel deals with how the Sholen and humans feud over their different approaches to making contact (or not) with newly-discovered intelligent creatures.This is a good one! Highly recommended.