Complex sentence: In the last operation (blowing up a Japanese submarine pen), the men correctly used drysuits; but they used old, bulky 3-cylinder open-circuit aqualungs, which make obvious clouds of bubbles, and useless for covert operations.
Split into simpler sentences: In the last operation (blowing up a Japanese submarine pen), the men used drysuits. They also used old, bulky 3-cylinder open-circuit aqualungs, which makes obvious clouds of bubbles, and useless for covert operations.