Hi Tim,
One thing to bear in mind is the compressive strength of the foam core. If you have any fixings or fasteners to make through the transom (outboard motor mounts; brackets/cleats/etc.), the common practice for foam cored transoms is to locally (to the thru' fixing) replace the foam with a hard pad of plywood or timber or even a metal pad which have significantly better compressive properties and won't crush when torquing bolts. The alternative being to use a large washer plate to sandwich the area and to prevent the core from crushing.
If the transom has no such fixings, you won't need to worry.