Raise the water, not the boat
How Does a Boat Go Uphill?
A canal lock does not lift a boat. It floats it, and then changes the level of the water it is floating on.
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Step 1 of 5
A floating boat sits where the water is.
A boat has no idea how high above the sea it is. It floats on whatever water is beneath it, so changing the water's level changes the boat's for free.
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Step 2 of 5
A lock is a box with a gate at each end.
Close both gates and you have a sealed chamber. Open a small valve in the upper gate and water flows in until the chamber matches the level above.
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Step 3 of 5
Gravity does all the work.
Water runs downhill into the chamber on its own. A lock needs no engine, which is why canals worked perfectly well two hundred years before electricity.
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Step 4 of 5
Every boat that goes up spends water.
A chamber full of water runs downhill each time a boat is raised, and it has to come from somewhere. A busy canal summit needs a reservoir feeding it constantly.
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Step 5 of 5
The gates are held shut by the water itself.
Lock gates meet at an angle pointing upstream, so the higher water presses them together. The harder the water pushes, the tighter they seal.
Boat lifts do the other thing: they carry a whole tank of water, boat included, up on a counterweight. The tank weighs the same whether a boat is in it or not.
The short version
A lock raises a boat by sealing it in a chamber and changing the water level, so the boat simply keeps floating at a new height.
Try it yourself
Float a cork in a cup and add water. The cork rises without being touched.