From OpenStreetMap Wiki
Access permission for boats
Legal access restriction for boats.
See also access=* for values.
- boat=unknown The access conditions are unknown or unclear. This is the default value for most features.
- boat=yes The public have official, legally-enshrined right of access, i.e. it's a right of way.
- boat=designated The route is marked as being a preferred route, usually for a specific vehicle type or types.
- boat=permissive The owner gives general permission for access.
- boat=private The owner may give permission on an individual basis.
- boat=no Access by this transport mode is not permitted, they don't have a right of way.