Complex sentence: The main role of the fleet was to ensure freedom of navigation along the Rhine, Scheldt and Meuse rivers and their tributaries, and continuing on up the coastline to Zuiderzee and the North Sea.
Split into simpler sentences: The main role of the fleet was to ensure freedom of navigation. The monitoring area included the rivers Rhine, Scheldt, Meuse and their tributaries, and continued up the coastline to Zuiderzee and the North Sea.