To identify water basins, I suggest : Waterway River lines should be mapped through lakes, estuaries and other large water bodies. Not in gulfs and oceans. As it has been told already, Rendering problems should not be fixed by removing waterways. Renders can take care how to render Lakes.
The Saint-Lawrence waterbody was given many times as an example. Note that the Great lakes and all the waterways that flow into the Saint-Lawrence are part of this basin, this until the end of the estuary. The waterway=fairway in the Great Lakes can be considered a derivative of the waterway=river with exception that it is located in the navigable way.
Note that estuaries (this is where tides are observed) are part of the rivers. If we would eliminate the estuary, the Saint-Lawrence waterbody would stop between Montréal and Québec in Lac Saint-Pierre. The gulfs and oceans start at the end of the estuary and yes should be ignored.
The coastline is an artificial definition to help manage oceans in OSM. The Great lakes and all the Saint-Lawrence used to be part of the coastline and later removed because the coastline was constantly broken and this made the lakes disappear ! Lets not adapt the waterbodies definition to coastlines.