You could use Johnstone's Aqua water based undercoat. Although not labelled as a primer/undercoat, it is actually suitable as a primer for interior and exterior wood, but an appropriate primer is required for metals. The problem with water based systems used outside is that they don't really have proven longevity. Of the few decent ones around, the Aqua system is one of the best. However, with plywood (and any other hardwoods), you may get resinous bleed through any water based system unless it is locked in with a suitable stain blocking primer/knotting solution.
EDIT:
I've used the Aqua system on a few hardwood outbuilding doors, which seem to be holding up well, but I did use a separate primer first.