Complex sentence: While planning for the attack on the U.S. Pacific Fleet, Japanese planners hoped that some significant units would be at anchor there because with Lahaina's deep water, those elements of the Pacific Fleet in all likelihood would never have been recovered.
Split into simpler sentences: While planning for the attack on the U.S. Pacific Fleet, Japanese planners hoped that some significant units would be at anchor there. With Lahaina's deep water, any elements of the Pacific Fleet that were sunk there in all likelihood would never have been recovered.