Something that I am surprised hasn't been mentioned yet despite being old is it depends on the type of salmon fishing you are going to do. If you are bar fishing, planning on hauling anything you catch up onto the beach and just using the boat as a transport from launch to bar, that can use a different kind of boat than if you are planning on casting and/or chasing after fish in the boat. It always cracks me up with the guys in their sled with the build in cabin where they only have a little door to get up front and inevitably the guy up front catching a Chinook and off they go, except no the guy is stuck up front and if the fish decides to head behind the boat, he can't see what is going on and it is a mad scramble.
If you are planning on chasing fish, especially Chinook, I would recommend a couple things to look for. Easy access with a pole with a fish tearing line from front to back. A non flat bottom as the bigger fish are going to give you fits with a flat bottom as they will come up under the boat and stay there and you won't be able to get a net on them.
If you are planning on casting from the boat repeatedly, make sure there is open space to be able to do so. I was laughing at three guys fishing from a sled where the front was unfishable so they were all in the back and they were playing musical chairs as there was only one place to cast from. A guy would cast from the starboard side, then take a couple steps to the port side, then would take a couple steps aft after the next guy cast so that he was in the aft port side of the boat to crank up and start his rotation over again. Even two people on the boat would have been a huge mess.