Serpent is a battle submarine. They have unmatched range and damage, and pose a very big threat to unguarded fleets. Their role can be compared to that of the Sharpshooter bot on land. They can deal heavy damage from afar, but are vulnerable in close quarters combat, and are easily destroyed by submarine killers.
Use them to destroy heavy ships and protect the sea from T3 amphibious units. However, do not rely on them as the only part of your fleet, as they will fail in a clash with advanced torpedo launchers or submarine killers.