-- Last Updated: Apr-14-12 9:40 PM EST --
Does not mean they all should leak!
I've been fortunate enough that the same spray deck fits on my WW kayak as well as on at least four of the sea kayaks t I have owned over the years.
On three of these, including on my WW boat that spray deck does not let any water in, excep through the tunnel. The seal is just about perfect. On my last kayak it does let a minimal amount of water in.
So it is a mix and match... If you find the righ t combination of spray deck and kayak you might get lucky and not get water in. And when I say it does not leak, I don't mean just during edging. It does not leak under lots of rolling and plenty of green water or foam going over it at speed. It is not unusual after a couple of hours of play boating with a few rolss and plenty of time with water over the deck for me to be completely dry: my dry top and PFD offer enough pressure on the tunnel so even that lets no water in. The only time I do get some water in is if I get trashed in a hole or roll while surfing a wave on a fast rapid - there the gushing water gets through the tunnel and gets me wet. But I don't get more than a cup or two in this way - not enough to come out through the drain plug on the boat (a light sponging out is all it needs to take it out)...
If i were to go in open water somewhere, I would not be comfortable with a spray deck that lets much more than a cup or two after hard use.