I recently purchased an Eagle Fishmark 480.  It says up to 60 degree beam with high sensitivity settings, however the manual says the transducer is a 20 degree beam.  I looked on the Lowrance website and the X135 says the same thing.  Does this mean the higher I set the sensitivity, the wider the beam? :-/

Where are you reading that 60 degree angle from. The diameter of the quartz element in the Xducer determines the cone angle, not the sensitity. I don't think you can even get a 60 degree angle in a single Xducer unit, 20 degree is the standard Lowrance Xducer. Even then it would only be useful in shallow water. In 20 Ft or greater, it would cover such are large area, you would have a heck of a time pinpointing the cover/structure you saw on the screen.

I was cleaning out the garge and just ran across the package lit on an old 480 I have an see what you're talking about.  Don't know where they are coming up with that 60 degree comment unless they are talking about the third echo you would get with a high sensitivity setting in shallow water, which is just about useless.  In a normal usuable mode, 20 degrees is what you're gonna get.