Marine Grade Plywood?
First of all Marine Grade Fir A-B plywood is only about 15% higher in cost than Exterior Grade A-B Fir plywood. It is worth every bit of that 15% increas in price. The other part of this is that if you can''t afford the 15% I can''t imagine how you are going to be able to afford to build a whole boat. (You can price this stuff at Harbor Sales who carrries good quality materials at reasonable prices. http://www.harborsales.net/index.cfm )
The hull of a boat is roughly 20-25% of the overall cost of building a boat. A reasonable grade of 3/4" A-B fir exterior plywood for your purposes would be close to $100 per sheet. A small beamy boat like you are proposing will use 12-15 sheets (more or less depending on how beamy and how thick you decide to plank). That is roughly $1200-$1500 just for hull planking. Add in another $1,000 or so for glass, resin and fastenings. If that is 25% of the boat you are talking about spending something like $10,000 before you are done.
There are a lot of very good boats out there for a lot less than $10,000.