Reduce number of workers for freshwater well

The freshwater well produces 3 water per worker, making it require 18 times as many workers as a main-island well, not counting the crew for the cargo ship and port. Shipping water in also demands fuel, unity, and suitable coastline, all of which can be in limited supply. Also, you'll want a significant stockpile since it takes a great deal of time between shipments.
All in all, I think there are enough complexities to make the freshwater well an interesting but not dominant choice, if the worker count is brought down to be more in line with the main island wells.