And, if you are a serious wader, you will plunge over your waist as the tricky floating knife bobs from the belt sheath and drifts unnoticed downcurrent. Been there, done that, and have several empty scabbards in a kitchen drawer. If you wade you don't need a fishing knife that floats. Regardless of length or buoyancy, if the knife is to be carried, make certain it is equipped with a rugged, firm-fitting sheath. A molded sheath of hard plastic that clips or locks the handle into place perhaps is the best choice.
Leather looks good but it can rot or stretch, or the stitching can fail.
A loose sheath is begging either a lost knife or an exposed blade. The latter is a far more serious situation. A long fillet knife is, in fact, an edged weapon. It must be monitored and used with care, but it is a traditional part of the total fishing experience. Many anglers fail to grasp this significance.