If you are doing the an Argosy Cruise, try the Locks cruise. The cruise shows you how the locks separate and connect the salt water of Puget Sound and the fresh water of Lake Union via the Hiram Chittenden Locks. You are also treated to views of the city and the "Sleepless in Seattle" houseboat community. After the tour, why not go to the locks and see how they look from above. There is also a locks museum where you can see how the locks work. The cruise is both educational for the kids and beautiful/romantic for you.
This is a picture of us on the cruise. This is when the locks are drained of fresh water and then filled back up with salt water from the other side. Look at the water level!