Here are some examples of important steps on a national level:
Netherlands
1st of January 2009 release of black water from any type or recreational craft was prohibited. Existing yachts with toilets must be equipped with holding tanks or equivalent capability as of this date.
United Kingdom
Direct discharge of sewage into inland waters is prohibited. Codes of Best Practice are strongly encouraged.
France
Recreational boats equipped with toilets and built after January 1st 2008 which enter maritime or inland ports must be equipped with an installation that permits either to store toilet water or treat it.
Spain
All vessels built per May 2004 or later must have a holding tank installed.
Baltic Sea
The Helsinki Convention for the protection of the Baltic Sea (HELCOM) recommends installation of toilet retention systems on all ships. The recommendation must be taken up by national authorities.