OpenStreetMap's database contains a lot of geoinformation, which is quite useful for planning a boat trip.
OpenNauticalChart creates a seamark-overlay from this data, OpenSeaMap makes this available online. Chart packages are also available for download, here.
A wide variety of chart styles and applications have been developed around this idea. Especially the Inland Navigator will find all necessary information about bridge heights, lock times and even automated routing throughout Europe.

To complete the maps, information on water depth is essential: openDEM shows in which sea areas free bathymetry data is available.

The project lives from many volunteers who continuously update data!