Raising water on the low side to the level on the high side simply requires joining the two sides together.
If sea level is their high side, I don't understand why they can't use this supply forever.
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.
The obvious solution is a sea-level tunnel a hundred miles long. Probably cheap. ;)
Chinese company got concession to build decade ago, but didn't do anything and then went out of business.
The original excavation was done mostly dry and massive by any standards: 27kt of dynamite were used.
I'm also not a digger, but 26 meters of dredging over a 13km distance sounds crazy expensive. You would have to also widen the valley to prevent landslides into the canal.