Do a market research
Know your competion. Identify what makes you different from your competitors. Find your weaknesses and work on them. Offer more than other (not talking about just cheaper prices.) Target 100% customer satisfaction as word of mouth is biggest advertising method. Once you think you are ready for promotion. You can start to use affiliate networks to promote your products and site. If you are in budget you can start with clixgalore and move to bigge networks.
I hope this helps.
Do a lot of link building via directory submissions, link exchanges, forum posting (especially in boat forums), etc. using proper anchor text for your links. This will help with your rankings and if you manage to get in the top 10 results in google for the phrase, 'boats for sale', you will get a lot of targetted traffic. Good luck!