I am certainly happy with the sealing of mine. Usually the only water getting in is what was lying around the handle before I open the hatch. After a vigorous six hour sail yesterday, with plenty of water flying around, when I tipped the hull down on the trailer until the stern touched the ground, I only got half a teacup of water out of the drainhole.
I do kayak trips with a lot of "normal" sea kayaks. Of course there are some leaking "normal" sea kayaks also.
But that is an exception from the normal state: ABSOLUTELY DRY in cargo areas in front and aft, if they are completely separated from the cockpit.
So in my mind it is possible to make good water tight hatch cowers. That is not that sensitve to sand and dirt.
That was my point when I started this thread.