While I was still living on shore I did a presentation entitled Marine Internet Sources. I uploaded it to http://www.scribd.com where you can download it for free. The presentation provides hot links on various boating topics (e.g. Safety, Charts, Weather, Radio) with the primary focus on US sources with some reference to Canadian sources. I hope to update it over the course of my travels to include information from other places I visit.
The classic source of information for boaters is The Mother of All Maritime Links which has been around since Operation Desert Storm from which it derived its name. Like Wikipedia it has far more links than you could ever use but if you are looking for something obscure you will most likely find it there.