My advice to you would be to continue looking at lots of boats. In a while, after seeing what is on the market and what people are asking for them, you'll get a rough idea of what things are worth.
When you find the right boat you will feel a bit of a twinge in your midsection - halfway between the first time you saw a cheerleader's skirt fly up and the last time you wet your pants.
If you have more money than the seller is asking you will go through the motions of negotiating but won't waste much time trying to beat the price down. You'll buy the boat and then go out and spend everything you have left on little presents for her.
If you have less money than the seller is asking you will negotiate like crazy, you will remortgage your house and send your spouse out to work the nearest corner for extra cash. When you finally have enough money you will buy the boat and then you will find more money from somewhere to go out and buy little presents for her.
Good Luck !