The North Atlantic Drift is a very powerful ocean current located in the North Atlantic Ocean. It is an extension of the Gulf Current. This current is warm, and as it moves towards the northeast, alongside the western coastline of Europe, it brings in much warmer and wetter climate. It makes the climate int he coastal regions much more stable, mild if you will, with pleasant summers, and mild winters, with high amounts of precipitation throughout the year.
If we compare it to other currents on the same latitudes, like the California Current, it is much different. The California Current is moving from the North Pacific, southwards alongside the western coats of North America. It is a clod current, and brings in colder and dryer weather in the parts that it affects.
The correct answer is - groundwater.
The freshwater is represented with only 3% of the total water reserves on the planet. Further more, most of the freshwater is actually frozen in the ice caps and glaciers on the poles and their close proximity, and that water accounts for around 87%. The rivers and the lakes do provide freshwater, but they only account for 1% of the total freshwater reserves, not to mention the fact that big portion of them are heavily polluted and can not be used for drinking at all.
The groundwater though, is much less polluted, is easy to extract and to use, and it accounts for around 12% of the total freshwater reserves, which makes the prime candidate for usage.