[ianburrell]

I think the thing you are missing is that raising the low side means that the next boat to use the lock going up has to drain the high water into the low canal. The Panama Canal has system to conserve that water but some water still moves down the locks both for raising and lowering.

The British locks depend on a source of water at the top of the canal. I watched a video recently about Canal Trust rebuilding lock reservoir. The water levels definitely can limit if locks and canals are usable.

The Panama Canal has a reservoir at the top, Gatan Lake, but the water is low. Sea level is never the high side, it is always the low side.