Hey Ryan,
I'm with Dave on this. I have heard a few horror stories on factory scarfs. I don't know what they use as a glue but I think it might not be well suited for the stresses of bending plywood. I bet Randy/Sandy could set you up with some pre-scarfed pannels if you did not want to tackle the job.
Mike. http://www.bakerwooddriftboats.com
If you are close enough to Randy Dersham's shop and don't have trouble hauling the 16' lengths you might check with him, he does the wavy stepped scarf joint for his kits. Good luck with your boat, I'm about ready to start my first boat also and this sight has been a great resource, check out some of the older posts, there have been a lot of discussions about scarf joints.
I have done the belt sander and it works but, you don't want to deal with the mess. Dust all over the place.
Use a hand plane. Sharpen the blade before you start. Practice on some scrap first. It's very easy. A plane is also much safer then putting 3 inches of circular saw blade into end of a sheet of plywood.
I also use a low angle block plane to clean up the blade marks on gunwale stock after it comes off the table saw. Much faster and easier then sanding them out.