For what it is worth -- I have owned 2 Fishers, a 16 foot w/25 HP Merc and a 18'6" w/ 115 Merc.
They were both good boats. I run the 18 now on inland lakes, as well as the great lakes and Lk. St Clair, but it is a deeper V. I don't know about the big water/fiberglass debate for that boat.
I'd love to have that 4 strk. Is it a 2004 also?
You might try to offer the dealer less, if you like the boat.
FYI - you can get an idea of what a used boat or motor range of value is at
http://www.nadaguides.com
Hope this helps and good luck