Complex sentence: The planet is mostly water so a submersible craft is the only means to explore it and catalogue the alien aquatic life while avoiding the syndicate (Eurocorp) that wants to thwart your mission.
Split into simpler sentences: The planet is mostly water so a submersible craft is the only means to explore it. Players are tasked with completing underwater missions and cataloging the new alien aquatic life while avoiding the syndicate (Eurocorp) that wants to thwart your mission.