There is always room for a price negotiation. However, there is not set amount or percentage profit margin on a used boat as all boat deals are different and how much they allowed for the boat on trade is relative to the unit that was being bought. I used to be in the business and we sometimes would give near the retail value if we had a particular boat that needed to be moved for one reason or another.
Cash is not what dealerships really want. Don't get me wrong, cash is good but dealerships get comissions on financing, extended warranties, etc. If I were serious about that unit, I'd look everything over very well and have a compression test done. I'd let them know that I was serious and make an offer of $14,400 and plan to have to give maybe up to $16,500.
You will have to be a firm serious buyer sitting in the sales manager's office to get this deal done right. You should also have a deal breaker in mind that you can toss in at the appropriate time in the negotiation.