Using salt water would destroy the freshwater ecosystem connected to the canal and contaminate ground water and irrigation system with saltwater which will wreak havoc to farming and freshwater supply for people that live in the area. Think about how hard it is to farm in the beach area where there is no fresh water. Most crops cannot tolerate high salt level. The canal is going through a highland so all those salt water will absolutely flow downstream to rivers and seeping into ground water. Imagine if an important water reservoir in your area suddenly turn into salt water reservoir. The whole region would basically be fucked to accommodate global trade.