I would try Craigslist first. It’s free and it works. I bought my boat and my truck from Craigslist ads. I can’t tell you how many I missed out on because when I contacted the seller just a few hours after posting they already had someone coming to look at it. Loosewire is right. Got to have pictures.
depends on the boat really, if you are selling a boat for less than $5k, i would just stick to craigslist. however, if it is much more than that, it may be worth it to get it on boattrader for more exposure.
also post it on this site and thehulltruth.com is a good one as well.