Complex sentence: Second, from Saint-Omer seawards, approximately, it is a navigable waterway connecting with the Canal de Calais leading to Calais and the Canal de Bourbourg leading to Dunkirk.
Split into simpler sentences: Second, from Saint-Omer seawards, approximately. It is a navigable waterway connecting with the Canal de Calais leading to Calais and the Canal de Bourbourg leading to Dunkirk.