Railcraft's water tanks slowly generate water depending on biome.
They produce a surprisingly large amount of water. Although the trickle is fairly low, it is an enormous quantity over time, especially if chunkloaded. The same principle applies to solar panels. Only one or two Railcraft tanks in a forest-type biome can handle most water needs for the early-middlish of the game, then you just add more as needed. I would not consider it balanced if you are trying to make water a resource; I've found it easily doable to run TE, IC and Mekanism setups with Railcraft tanks.
However, without Railcraft tanks, it is still very easy if you drain a large lake or ocean. As I said before, there is a deceptively massive amount of water in the world. To put it in perspective, if you take a 100x100x25 slice of an ocean, it would yield 250,000 buckets of water. IIRC it takes 1 bucket of water per operation in the IC2 ore washer. So, you could process 250,000 ore with this amount of water. If you drain large lakes or oceans, it will be similar to pumping the Nether; essentially infinite except you have to move the pump every so often. To actually have a challenge you should only use small bodies of water, such as ponds and small rivers, and try to use the water intelligently. Think of it like those little pools of Buildcraft oil.