Depends on how many cubes of water per second are moved (since only the “front” and the “back” are actually changing). Just be aware that the Game is gonna be highly multithreaded, so Issues like that would at least not slow down your game too much normally.
Though there wont be any Chunk Loading shenanigans like in Minecraft, meaning the entire Island/Region you are at is “active”, when it comes to Blocks, no matter how far you are away. Mobs and other AI controlled things will likely stand still when nobody is closeby though.
This makes it very important that most things are doing their Stuff while “inactive” to reduce the amount of “active” things the Game has to care about. For example Crops will only grow when someone is closeby to see them grow, but they will get a huge growth spurt before they load into view to compensate for the off-time.
NPCs do count as “witnessing crops” but in that case they only check for fully grown Crops so there are easy special cases to be made to update Crops only when they are supposed to have reached the fully grown State, kindof like a huge timer.