Complex sentence: The round ships with oars that were used for Mediterranean sailing were not useful in the rougher Atlantic waters and the maneuverable Viking Longships lacked cargo capacity and could not use their sails against the wind.
Split into simpler sentences: The round ships with oars that were used for Mediterranean sailing were not useful in the rougher Atlantic waters. The maneuverable Viking Longships lacked cargo capacity and could not use their sails against the wind.