Thats false, the rainwater never fell to the ground and teleported to the ocean.
Just consider the nearest rivers near you, now imagine someone diverting the river upstream to form an artificial lake (let's ignore the other environmental impacts of that for the sake of argument), to enable formation of something akin to the Panama Canal.
I'm sure you'd have certain reservations and complaints about the local loss of access to a now diverted river.
Then you come to understand that it's not even necessary for this canal to continue consuming fresh water for the operation of the locks, but hey there are some economic advantages of the shipping shortcut.
Then you learn shipping capacity will be throttled because the canal is not the only consumer of the rainwater, and El Nino threatens a big drought. The ships will take longer routes outside of your economy, and wasting fossil fuels, further exacerbating global warming.
All of this turns out to be a false dilemma, its perfectly feasible to have water locks operate and raise and lower ships without consuming fresh water.
It's orders of magnitude more expensive to desalinate sea water than it is to simply prevent fresh water from being needlessly wasted on operating water locks...
Even if you have a "surplus" of fresh water, its better to let it seep into continental ground and let water tension distribute it throughout the soil, so the soil can evaporatively cool during prolonged heat waves.