Water – Should you coat exposed roofing nails with neoprene washers

Plywood absorbs water and is not a sealing material, period. You can use it as base for ruberoid roof, but not as the layer exposed to the rain.

When exposed to rain, wind and sun plywood will cycle through absorbing water and drying out and this will wear out the most outer layer in no time so that it cracks and the damage then proceeds to the next layer of plywood.

In case you want a cheap roof for a tools storage you can cover the plywood with one layer of ruberoid - it will last something like 5-10 years and the plywood can then be reused.

I wouldn't worry too much about it. The plywood is not typically supporting a load of more than the paper and asphalt shingles that were placed, even more so if you have a considerable pitch on your roof to where it would be difficult or impossible for a person to walk on. It basically serves so that you have something to nail the shingles down securely.

Most newer homes use plywood, I have an older home that used what looks like rough cut deck planks and those are mostly still original. The last time I had my roof replaced, they had to change out a few boards that were terribly dry rotted and some that were badly damaged from previous water damage by the oh-so responsible homeowners who had the home before me. Other older boards look noticeably damaged by the most recent nails but they continue to hold down the shingles and I would still feel safe to stand on my roof.

As long as the shingles are doing their job then the plywood will not be exposed to any water, and as long as your attic has good ventilation during the summer time then you should have nothing to worry about for the next 15-20 years at least.