Fly lines for specifically built for lake fishing will be sinking. By far the most popular lines are "intermediate" meaning they sink at 1.5 - 2 inches per second. Your general purpose floating lines can work for dry flies, shallow water, or indicator fishing with Chironomids, but to get down in the water column where the mature trout are often feeding you'll need a sinking line. Depending on your fly size, there are several options. Anglers fishing sparse leeches, swimming nymphs, and Chironomids typically like a classic taper without an aggressive short head. Some anglers might be throwing large streamers and want a faster sinking line, if this is the case, look for a fly line with a stout head capable of turning over large flies.