Re: Best knot to attach a painter to a mooring ball?
A bowline is a great knot, but way overused. It is not a secure knot, particularly when subject to loading, unloading forces repeatedly. That's why you'll never see a climber using it. Some of the options listed above are great, and if you do choose a bowline, definitely secure it further somehow, but I would avoid it altogether for that purpose.
Another option is to use a double figure eight, like climbers do. It's a little harder to undo, but not that bad. It is much more secure, which is even more important when using polypropylene line, which tends to be slick and not hold knots very well.