I'll second the opinion on Vanish (the negative one that is). The first fluorocarbon I used was Vanish. Never again. It was very inconsistent. One leader would work ok, the next from the same spool would break or delaminate. I use Seagar now. If you are using it for leaders, you are not really using that much at a time. Compared to everything else in fly fishing, it is actually pretty cheap.

If you are using a floating line, I would recommend a mono leader and a long fluorocarbon tippet. That's what I use most of the time in saltwater.