To find boats for sale by owner on craigslist.org, simply select "boats" under the "for sale" section and then select the "BY-OWNER OPTION". There are always listings for boats on the "for sale" section and they give you the option to buy from an owner or from reputable dealers.
Once you have accessed the section, you can view boat listings from owners trying to sell their boats. The listing usually includes the name or type of boat, the particular model year, the location of the boat, a price and a few pictures. The seller may or may not include his personal contact information.