The 2 well-known brands of rot-resistant plywood, which are KDAT (Kiln Dried After Treatment) are 'Greenwood XL' and 'Perma Panel'
You can check the APA (American Plywood Association) ratings for plywood. The glue is the same for the 'Exposure1', 'Exterior', and 'Marine' rated plywood's. The differences are mainly in the quality of the veneers that make up the plywood, 'Marine' being the best quality, 'Exposure1' being the least.
There is a thread currently on-going about plywood for boat construction, if you really want to become confused.
For a transom, I wouldn't sweat it too much and get what you can in your area that has at least the 'Exterior' rating.