The three most important knots that I use are: The "bowline", the "Clove hitch", and the "sheet bend", in that order. Pulled my neighbors boat off the rocks last Sunday, and he used Reef knots to join dock lines for the pull. He had to cut those knots off to get the lines apart. Had he used a "Sheet Bend" they would have easily separated.
International guild of knot tyers

I try and cary long enough line to not have to join them together My most used knot is the PROPER cleat hitch!!!! I am amazed how many people do it wrong and do not realize that their boat is about to float away!!!! We came into a dock just last weekend and found a bout about to float off. Bowline is probably my next most used fallowed by two half hitches.

Jon

So, you mean to tell me that you can't just tie them like your shoes?

Erie, I couldn't get the link to work!
Ditto on the bowline, don't know the names of the others I use.

Fixed the link

Erie Desire:

Thanks for the link to the knots. I use the same as you do for the most popular and I completely agree with Audiofn about how a “boater” can’t tie a cleat hitch the proper way. They really should just stay at home where they belong! Although they are entertaining to watch along with the ramp people that don't know what's going on!

I'm KNOT sure which ones I use.

heres another handy link or boat knots. each one has an animated demo for the most usefull knots.these are the basics every boater should know. pretty cool.

audio , you aint kid'n. its sad how few people know the proper way to wind a cleat. most bone heads i see just wrap it around a few times cross it once or twice than wrap it until out of rope "there . . .that uoght'a hold it"

boat knots too

bowline
sheep shank
square knot

A "good knot" will never come un-done, but can always be untied.

Most of the knots that I make receive their name when someone is trying to untie them. My most popular knot is the "WTF is this" knot followed by the one in the anchor rope bucket which is called a "single overhand cluster f@#$" knot. (Just kidding).

Almost ashamed to say it but other than tieing down the anchor or mooring rope to a cleat, I'm clueless to knots. Some good links though, appreciate it.