I believe the Expedition has a lot of similar parts to the newer 400 trawlers. This is what the manifold on my 400 looks like, and the circled valve is the one for the summer kitchen sink. On mine, this was the only valve, i.e. there wasn't anything under the summer kitchen to shut off the water to the bridge so unless you have another valve under the sink, you should be getting water. A couple things to check, obviously the fresh water pump needs to be turned on. Make sure the strainer by the fresh water pump is clear.
Check the aerator (if present) on the upper sink (mine doesn't have one) Try turning the shut-off 90 degrees just to make sure you are not misreading the position of it. Popping the connections apart is not too hard, except for the difficult access, so I'd disconnect something under the kitchen and see if I had flow then, which would mean something upstairs must be plugged up.
My ice maker wasn't getting water either so I disconnected the tubing to the solenoid valve and turned on the water...got flow, so I concluded the solenoid valve was probably defective or some other sensor in the ice maker was malfunctioning.
Good luck and congrats on a fine boat!
The man who goes farthest is generally the one who is willing to do and dare. The sure-thing boat never gets far from shore. Dale Carnegie