Kapowsin CAN be a great lake... Just be sure of the duck you are shooting, make sure you aren't shooting some sick diver. They look real pretty, but smell like crap and taste even worse.
Good areas on the lake are [from the dock]: Straight left into the reedy, overgrown area you'll see. Bring some camo netting or something to make a little blind in the water. The same works if you are willing to travel all the way down RIGHT of the dock, to the end of the lake. It's quite a ways... like two miles.
What seems to happen at this lake (for the most part) is the hunters on the left will push the ducks down to the hunters on the right. Usually, if me and my bro go, we're the only hunters on the right, and have quite a bit of success.
An area we found to work real good every now and then also is the little canal behind the island, straight out and to the right of the dock. we liked to work our boat through there and jump whatever was on the way, then set up our boat in the bigger, circular area towards the right exit. Kinda hard to understand now, but you'll probably pick up what I mean when you get there.
Most of all, watch out for stumps! Many a boat has sunk in that lake due to those logs. They are above and under, so keep someone at the front keeping a sharp lookout into the water.
Have fun! Any more questions just send me an email!
Curtis