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/