There is difference in opinion regarding the use of relative velocity instead of particle velocity alone in the estimation of drag force or power. In the present study, a tethered spherical float which undergoes oscillatory motion in regular waves is considered for the analysis. Float velocity is computed through dynamical equation of motion and particle velocity using linear wave theory. The method of computation is briefly described. The results show that variation of particle velocity with respect to wave period, wave height or water depth is small compared to the variation of float velocity with respect to the same parameters. The difference between float velocity and particle velocity is considerable and in such cases, relative velocity instead of particle velocity has to be considered for drag force or drag power computation. It is suggested that float velocity must be at least twice the particle velocity in order to use relative velocity in drag force or drag power estimation.