Bigkat650 said:
Exterior grade plywood is where its at. Same glue that is used in marine grade, but the difference is it's not guaranteed to be free of voids--which is not worth spending 2 or 3 times the cost of exterior grade. Just get it to the thickness you want, then coat with several coats of a water-resistant finish and you'll be all set for years to come
The only problem with that is that you can't usually get the thickness you want...most boats need a couple layers of a thickness...for example two layers of 3/4" then you laminate (glue) those two layers together to get the thickness you need.
Also if you want to be set for years to come you can't just use any water-resistant finish, they are not created equal. Do not use spar varnish or spar polyurethane...if you are using spar use spar urethane.
As the past two posters mentioned, the exterior grade is perfectly fine, but I prefer a hardwood ply. The reason I prefer that is that my transom is entirely exposed in front and a nice oak (or other hardwood) is going to look a lot nicer - also it is harder than typical exterior ply. If you have an aluminum front that covers your transom or you plan on painting it than by all means go for the exterior grade - it will do you fine.