That is a difficult question to answer. In my situation, my boat was being offered for $13,500 and I was not willing to finance that much due to what I wanted my monthly payment to be. I offered what I could afford which was a few thousand less than the asking price. I was honest and told him that I could only afford that much and he accepted my offer. His boat had not sold all summer, so he was ready to come down on price.
Each situation is different. Offer what you feel the boat is worth after you have looked/driven it. $10,900obo means that he will take his best offer.