You can get braided (not three strand) polypropylene at Home Depot. It
will work and tie just like regular braided line, but floats. Comes in
colors like green, blue, etc. If it's out in the sun all the time, I'd
replace it yearly; it'll still weaken quite a bit in constant
sunlight.

If you want to use that yellow stuff (three strand) splice it and
forget the knots.