Let me see if I understand displacement correctly....
The Falkirk Wheel in Scotland is the weight of each gondola the same whether it has a boat in it or not?
What if the boats are much heaver on one side?
Bear in mind that much of the physics occurred way before any of our different-sized boats arrived at the Falkirk Wheel (in this instance).
To clarify, a boat displaced the volume of water equivalent to it's own weight when it was first launched, and although it is displacing water all the time it remains submerged in it, all the boat is really doing is swapping places with it's own weight of water, as it moves through it.
Therefore, going back to the Falkirk Wheel, if a 7.641-ton boat (for example) goes into a gondola, it will push 7.641 tons of water out of the way.
The volume of displaced water (weighing 7.641 tons), will be equal to the amount of boat that is below the water surface.