The Nautical Institute Mars Reports – has a weekly report of maritime accidents, incidents and near misses with commentary.
The International Maritime Organization – has the latest on meetings and upcoming rules and regulations.
The Electronic Code of Federal Regulations – Important for Mariners from or calling on the United States – contains the Laws and Regulations applicable to U.S. Vessels and all foreign vessels operating in U.S. Waters.
United States Coast Guard National Maritime Center – to keep up to date with new policy changes and regulations.
OCIMF Ship Inspection Report Programme (SIRE) – the recommendations for Tankers – but lots of sound advice for all vessels
National Transportation Safety Board – Investigations into transportation accedencts in or involving the U.S.
gCaptain – The latest maritime news.
I will add more links as time goes on.
If you have suggestions please add them in the comments.