[willy]

They do well on my boat, if you check my gallery you'll see they sit about two inches above the water line. Even so if you got guys standing in the back and your rocking they let water in and then it flows back out. They all do. I've been on big sport fishers that do. You can add a one way drain like I think Hammer did and that should work but they will not be as free flowing and you will have to make sure nothing clogs it up inside.
What I do is before I launch I put in three plugs and they all sit together in the transom compartment. One is the standard screw in garber plug of bronze, the other two are the rubber and metal type garber plugs, the kind that you put in the hole and flip down the toggle to snug it. Those two go in the scupper holes, perfect fit. If I get into any snotty stuff I just pull them right out (two fingers) or if I am washing fish crap off the deck. Etc. If it is just normal fishing and playing, even in a heavy chop I never get any water in the boat so they just stay in.
If I got any water from super models climbing in and out from swimming tubing etc I just pull one of the plugs when I start moving forward and it all runs out in two seconds.