Curious Tides by Lacelle Pascale

Pitched as a YA Ninth House meets The Starless Sea, this crossover dark academia fantasy follows a young mage who believes her secrets drowned along with several of her classmates—until their not-quite corpses start washing ashore. With the help of a reclusive student, she must uncover the truth behind the drownings—and the secret society linked to them—before the sinister magic that claimed her classmates claims her next.