Sir L had naval superiority, in fact he had naval superiority the entire game.
Prepare did not and was using T2 torp bombers to fight off the navy, that and building naval yards to get out a couple subs before Sir L destroyed the factories.
Sir L must have been pretty confident because the entire time he ignored is eco, and prepare (who was being attacked and losing units en mass, and being shelled by t2 destroyers/cruiser) got the first T3 mass fab.
Even then he could not get a foot hold in the sea (in a real sense) and prepare eventually eco slaughtered him with air power. first by gaining air superiority completely then leveraging that to destroy Sir L's T3 battleships, and then with Soul Rippers to come in and molest his base and Sir L quit.