Marine poly... but it's expensive and has horriffic vapors.

APA says use an oil-based primer, paying special attention to edges (and knots if you want to prevent bleed-thru), then use a good quality acrylic latex exterior house paint over the oil-based primer (makes sense).

The primer doesn't take all that long to dry. The acrylic latex house paint can take a while if you put it on too thick, and in my experience, it can be "sticky" for a long time - like if you stack plywood sheets flat on top of each other. However, it works rather well. Marine Polyurethane is the only other exterior stuff I've used on plywood that really holds up to wet, UV, etc. but I no longer care to bother with the darned fumes from it.

HTH

Tom