What kind of boat? Airboat are a snap...jet boats should be okay..I have seen several launched there. I put my airboat in there all the time (down by the tree). I don't like the fast water down by the new bridge when I am by myself. PM me if you need any more info....
With that big of boat, I would recommend launching at the New Glenn hwy. There is much more water (since it is below where the Matanuska meets the Knik) and if you are planning on going upstream from the old bridge, it isn't much of a run between them. The current really isn't all that fast, especially if you hit it near high tide.
We have launched at the old Knik Bridge or decades. The trick is to find a deep spot, usually by the tree, but keep an eye out for the thick mud. It will grab you really quick, and its unforgiving. When the water was deep enough we used to unload right in the mouth of Bodenburg, and if it is really high we unloaded in that pond to the right after pulling off the highway.
In previous years the traffic wasnt that bad, so I wouldnt be worrying about accidents. Worry about someone breaking out your window.
We also used to unload at the new Glenn bridge. This was much easier and not very difficult. Unload down river as far as you can. This side offers much more gravel and less sticky mud. The current isnt that bad.
On another note, I watched a guy try to cross Jim Creek with a jet boat. The driver was standing up working the tiller and he had two people sitting in front of him. He hit a sandbar at about half speed and came to a dead stop. All three went flying to the bow. It looked painful. The boat crept forward just enough to make it to the other side though. I laughed just because no one was seriously injured and that has happened to us many times. Some of the most memorable white knuckle moments of my life has ocurred on the Jim Creek flats in a boat.