There is a Portland cement product called Thorough Seal (spelling may be off) which when applied to the block will seal it. If the basement is below the water table, it won't hold back the tide, but it does a good job of sealing out moisture. It comes as a powder, you mix with water and apply with a big stiff brush. I have had good results with this stuff.

If you have a serious water in the basement problem, let me know... that is a different animal.

As for studs, generally when building a wall, you go 16 in on center with 2x4 's and can go 24 in on center with a 6 in wall using 2x6's. The advantage of the 6 inch is you can put more insulation in there. For a basement wall, which is not bearing any weight, I would recommend the 2x4's (some even use 2x3's) set on 16 in centers and build the wall out from the cement blocks, so that you can still use the 6 in insulation. Since the cement block wall forms the other side, this works very well.