This is vague about your intentions for the simulator. Are you trying to make a to scale version of the french/ Netherland and Belgium ocean and shoreline? or are you trying to recreate a patch of ocean with these countries shore lines in the back ground? Are you intending to be able to get off the boat and walk on the terrain or is it just for contextual and aesthetic purposes? Your intentions and use case for the final piece will inform the best way to crack this nut. So please give a detailed descriptions of what it is you are trying to achieve.
You can use world machine if you are trying to create generic landscapes or simply youtube “Import real world locations into UE4” there are a few good tutorials. - You will probably also benefit from watching the world composition live stream ue4 did. https://youtube.com/watch?v=FrcNClb6KUI
Thank you in advance for your answer Malach93. “I want to build a shipsimulator in UE4.20. For places in Europe eg: the Netherlands, Belgium, parts of France.” I want to create the rivers, lakes,buildings of cities, locks. The terrain is not needed for walking only for boats.