1. Sea Duck endorsement.
2. What gear is needed? Assuming your string of decoys and appropriate watercraft to get to spot and hunt out of? Paint up old floaters and turn them into sea ducks. Once you do it a few times, you probably won't make them a main focus unless you have buddies trying to fill their bucket lists. Long lines and droppers. Anything cheap and heavy for anchoring both ends.
3. Any recs on watercraft? Does it have to shield it with camo and such? You could shoot scooters out of a ski boat. Anything with higher sides as the water can get rough. You can clip on a gray tarp that wraps the boat and hangs to the water. Heavier oil tarps work great painted gray. Or a gray boat. I used to camo mine up good, then I got lazy and just sat in my boat with decoys down stream and to the side and I still would shoot my limit.
4. Any sea duck hunting in southworth area or south Sound? Get out and scout, stay out of no shooting zones and off of private tidal areas.
5. Can you basically hunt anywhere in Puget Sound or good canal since it basically a state park? No, you will have to ask DFW and maybe some locals may help you out. You probably won't get to much help just posted on the internet as you burn your spots that way.