Is there a nautical map equivalent of osm or organic maps? One that emphasizes waterways by drawing them thicker when zoomed out like regular maps draw roads thicker? Plan routes over the water? Even google maps lacks a nautical layer.
The OSS tool for nautical charts is OpenCPN.
it depends what type of waterways you’re after. for the sea, https://map.openseamap.org/ is very good (but no route planning sadly - for that you’d need opencpn and some charts obtained. For inland waterways i can only speak for the English canals, for which i recommend https://opencanalmap.uk/ which uses data from both OSM and the Canal and River Trust. Again, sadly no route planning but for that I use https://canalplan.org.uk/