Complex sentence: If attacked by a shark, Lipski would have thrashed about, and a "giant octopus attack" would have been impossible because the only species of octopus strong enough to drag down an adult human, Enteroctopus dofleini, is restricted to the Pacific Ocean.
Split into simpler sentences: If attacked by a shark, Lipski would have thrashed about. An octopus attack would have been impossible because the only species of octopus strong enough to drag down an adult human, Enteroctopus dofleini, is restricted to the Pacific Ocean.