Float test is not 100% valid with lightsabers, it helps as a starting point, but you cant distinguish real from repro lightsabers based on the float test alone. Depending on the lightsaber type, flat tips, round tips, repros, they all perform differently depending on the type of saber.
Your description matches an original flat tip lightsaber, floats, and can also sink when pushed or droped in water. I guess you have a flat tip if the figure is a farm boy, based on your description. Round tips are a completely different story, there are repros that are a perfect match in every way, plastic colour, float, glow under UV just like an original. Some farm boys can also have a round tip saber from memory, (I could be wrong), these particular round tip repros are almost impossible to tell apart.
To give an accurate opinion, a photo, in natural sunlight is best.
Look for letters on flat tip hilts.
Round tips need to be very closely looked at, and possibly you wont get a definite answer from a photo.
That would be your best way to start.