Complex sentence: The Soviets' weakness is at the sea, their only offensive naval unit is the submarine which exists to counter Allied warships and gain intelligence, it is normally invisible except when surfacing to attack where it is vulnerable to destroyers and gunboats.
Split into simpler sentences: The Soviets' weakness is at sea; their only offensive naval unit is the submarine, which exists to counter Allied warships and gain intelligence. It is normally invisible except when surfacing to attack, when it is vulnerable to destroyers and gunboats.