Between levels.
A boat cannot climb a hill. A chamber of water can carry it there. Work the gates and see how.
0.0 mRise above the lower canal
The boat is ready to rise.
Both gates are closed. Open the upper sluice to admit water into the chamber.
≈
Water seeks its level
A sluice admits water from the upper canal until the chamber reaches the same height.
⊣
Gates hold the difference
The upper gate stays closed while levels differ. It opens once the two surfaces are equal.
⇅
The boat floats throughout
Buoyancy keeps the hull on the surface as the water rises or falls beneath it.