Boating Knots: Covers several knots commonly used in boating.
Boating Magazine: Pictures and instructions on how to tie a few knots.
Marinews Boating: Covers common boating knots including the Cleat wind, Clove hitch, and Wagoner's hitch. Animations and common uses.
Shooting Star's Marlinspike Page: Covers common boating knots including the Anchor bend, the Carrick bend, the Buntline hitch, and several variations of the Bowline. Animated illustrations and step-by-step instructions. Table showing the relative strength of the knots.
Uspowerboating.com: Boating Knots: Pictures and videos of a great variety.