Setting is straight out of the Lovecraft short story The Temple. A submarine gets crippled and stuck on the ocean floor near an eldritch temple. You play the crew venturing forth in diving suits to explore. Your crippled sub acts as a base of operations. Bring back materials to slowly pump out flood water and repair the systems, unlocking functionality just like an XCOM base.
All the XCOM features during the missions are there. It's aiming for more of an atmospheric adventure vibe, which I'm fine with if it can keep the interest up.
Graphics are a tad too bright and cartoony for the tone, but I'll live.