Floating line with weight - even lots of weight - is no substitute for sinking line.  Trust me, I learned it the hard way.

I recently tried fishing for White Bass in a central California lake.  The fish were holding right on the bottom, probably gaurding nests, and I only had floating line with me.  I added weight and used heavy flies with my 7-weight rod with a 5-weight reel to help manage the heavy rig, and that part worked well.  What didn't work was that every cast lost about 20% of its distance due to the floating fly line.

So, to start - the cast:

Then I would let the big streamer sink.  The water was clear enough to watch at least the first half of the decent.

Eventually, the fly line would begin to sink as the fly neared the bottom.

Once the fly got down, I would strip it in.  Using the common technique of two quick and short pulls followed by a pause, I could watch my fly follow the fly line towards the surface.  The amount of quality time my fly spent in the strike zone was seriously limited by not having the proper gear.

Sinking line on the other hand, sinks uniformly which maximizes your cast length and time your fly is in the strike zone.