Finish plywood for closet organizer
I used to prefer oil based finishes, but the water based ones have gotten so much better over the years that I will now use them instead. For a simple closet organizer I suggest 2 coats, lightly sanding after the first coat.  The main problem with water based finishes is they tend to "raise the grain" of the wood on the first coat, which requires light sanding to remove.  Almost always the second coat doesn't.  And if you aren't yet satisfied with the 2nd coat, another light sanding and third coat would almost certainly be sufficient.   

But to be succinct, I really think for your purposes 2 coats of a quality, water based Poly would be fine.