can they not take water from rivers before it gets to sea???
or is that a really thick question??? 
Bristol Water(or it's forerunners) did just that. There is a canal between Sharpness and Gloucester which is fed by the river Severn. (Which of course is fed largely by the rainfall on the mountains of Mid Wales)
So about a hundred years ago (approx) Bristol Water paid for and continues to maintain feed from the Sharpness Canal into Bristol - a distance of about twenty five miles. It also has reservoirs at Chew Valley and Cheddar.
Birmingham also did well about a hundred years ago in building the Elan valley reservoirs in Mid Wales and pipes water by gravity 73 miles to Birmingham.
It's a shame that Victorian and Edwardian foresight and municipal building isn't done now.
Lecture over!