One of the most common and annoying problems occurring on the water is the tip of a floating fly line no longer floating. Some lines float better than others but all have less flotation at the tip due to the construction of the fly line.
All lines have a core and a coating. Most floating lines get a little flotation out of the core but most of the flotation comes from micro-balloons mixed into the coating and the hydrophobic coating added to the line. Since most modern lines taper, there’s less flotation at the tip of the line because there’s less coating. The core of a line is the same thickness from end to end causing all the tapering to be done with the coating. With the taper getting thinner near the fishing end of the line there are less micro-balloons causing less flotation. This is a bit ironic since the tip is where we need the most flotation.
A line with a welded loop at the tip often floats better than one without a loop due to the increased surface area of the loop (much to my dismay as I dislike loop to loop connections).
The other common cause of a fly line sinking is dirt. Dirt clinging to the coating of a fly line adds density, causing the line to sink. Keep your line clean and it will help flotation. Soaking a fly line in lukewarm water with mild dish soap will help loosen up dirt and allow you to clean the line. Soak the line for a few minutes, then wipe a clean cloth over the coating. Repeat this process until the cloth is no longer taking dirt off the line (evidenced by a clean cloth after running the fly line through it). Scientific Anglers sells a micro-abrasive pad that can be used to remove dirt from their fly lines but be careful, it isn’t safe to use on all brands. The SA pad can be used dry.
Keep your floating line high and dry to make your time on the water more enjoyable.