Simon,
I used 9/16" staples. I also bought a new staple gun when I had started which worked fantastic. It is called the "Power Shot or Big Shot". It is the type that the handle you squeeze is reversed from the old style. The pressure is applied at the end where the staple comes out the bottom. This feature is very useful when you are bending and holding strips. It uses T-50 staples which are very commen (at least in the Northeast).. It should be availible at any home store or builders supply.
Good Luck on the new boat;
Timothy Mulherin