Regarding the question of the canal locks:
The larger boat displaces more water, so when the water level is high, there is less water in the lock, and more boat. However, when the water level is lower, the lock is not empty. The boat is still floating, so its displacement is the same.
Suppose the lock holds 10 million litres without a ship in it, and 3 million litres when the water is low. The displacement of the kayak is about 100 L (it is about 100 kg with the passenger inside) and the displacement of the ship is about 1 million L (it weighs 1000 tons). The contents of the lock will be:
Water high Water low Difference
Liner 9000000 L 2000000 L 7000000 L
Kayak 9999900 L 2999900 L 7000000 L
For either boat, the amount of water used to cycle the lock is the same.
Fortunately, the question is a lot more interesting than that. Suppose the liner wants to go down, the water level in the lock is initially high, and the lock is fed by a reservoir at the top. When the liner moves from the reservoir into the lock, the reservoir loses 1 million L of ship and gains 1 million L of water (which flowed out of the lock when the ship entered). After the liner decends, there will be more water in the reservoir than there would have been if the kayak had decended. Now let's suppose that another liner wants to come up. The water level is still low after the first liner exited the lock. The ship enters, pushing out 1 million L of water. The gates close, and 7 million L enters the lock to cycle it. The ship exits, but there is less water in the top reservoir since the lower water source contributed less water, because the ship was taking up space.
As long as the same displacement tonnage of boats goes up as goes down, it works out that there's no net gain or loss. If you have a lot of ships going down and a lot of kayaks coming up, you keep more water in the top reservoir (in the short term).
A couple more issues: small boats share the lock, so instead of using a lockful of water for one kayak, you divide that water by all the boats sharing the lock. On the other hand, if you're carrying cargo, one big ship can carry more cargo than a lockful of little boats, so the amount of water used per unit of cargo is less for the big ship.