As a lapsed marine zoologist, I couldn’t help but love A Darkling Sea. It has aliens, intrigue, desperate missions, and it all happens underwater.
On a moon orbiting a gas giant, a human science station sits in pitch-black water. They’re studying a semi-primitive alien species, but when a dumb-ass scientist gets himself killed by the curious aliens, another alien species visits the science station and tries to take over. Chaos ensues.
Recommendation: Get it at the library. It’s an excellent read, but not mandatory for a bookshelf.