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.

Some lock flights have pumps to keep the ponds at the top full of water because they are near the highest point in the area so they can't refill naturally fast enough or simply to avoid depleting the upstream water source. See, e.g., https://en.wikipedia.org/wiki/Caen_Hill_Locks

The sea level is the low side. The lake is the high side - which works great until the supply of lake water dries up.

The obvious solution is a sea-level tunnel a hundred miles long. Probably cheap. ;)

Sea level is a low side on both ends of the channel.