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The docking mechanism makes a water-tight seal between the end of the box and the side of the chamber by the upper canal.
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In this enlarged drawing, the 'outer' door in the chamber wall and the 'inner' door in the end of the box can be seen. They are still separated by a water-filled space at this stage.

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The water is drained out of the space between the doors (Weldon probably fitted a tap for this purpose) and air is allowed to take its place.

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The hydrostatic pressure due to the water above the box forces the box into contact with the side of the chamber. Weldon's patent also shows a lever with a rachet mechanism (rather like a car handbrake) for locking the box firmly in place to prevent leakage.

There was probably a frame to ensure that this pressure did not trap the outer door.

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With atmospheric pressure both sides of the outer door, it can now be raised, allowing water from the upper canal access to the inner door. The forces from the locking lever (not shown) and hydrostatic pressure from the higher water level in the chamber, keeping the box pressed against the side, are sufficient to ensure there are no leaks outwards.

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With the outer door fully raised, the inner door can now be opened.

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The inner door is opened allowing communication between the upper canal water and the inside of the box.
It is important that the levels are exactly the same so that there is no inward or outward flow which would upset the overall buoyancy of the box.