My experience with the "self bailing" center console fishing boats is that
no scupper or check valve arrangement works very well in the one way mode,
however they all seem to work fine in the two way mode. There the little
floating caged ping pong balls, rigid flapper with springs and various
other flappers that are installed. Most on these decks are about 6" above
the water until everybody runs to the spot in the rear of the boat where the
big fish is, then water comes in. My take on this was to go with really big
scuppers (3" dia) with Salisbury exhaust flappers and flappers work pretty
good against a strong backwash or wave action, but without some force from
the water trying to get in the boat, the water just leaks around them.At
least big ones drain fast when you go back to normal trim. So my guess would
be gentle swells would leak and bigger more forceful swells that would tend
to snap the flapper shut would wash over your hatches and unless they are
gasketed really well (perfectly) you will get more water in than out which
will keep them pretty much filled. So unless these are true waterproof
latches the 3/4" drain with or without a one way valve will get overwhelmed.
If you change to a much larger drain at least it will drain fast weather
water backflows in from the one way drain valve or past the "waterproof"
hatches. So, go with something bigger and expect some back flow. If you have
any breakable stuff in these lockers and the fill with water and the boat is
in a seaway sh-t will break. Also 3/4" size might be prone to get plugged
with the stuff that normally plugs drains on boats. The only problem I have
had with big scuppers is that once anything hits the deck it seems to
immediately go out the scupper.
So to sum up, go bigger and good luck with the one way mode, plan for two
way.

--
Ron White
Boat building web address is
www.concentric.net/~knotreel