In this paper, we propose and use an experimental method permitting the measurement of optical depth of a selfabsorbed spectral line emitted from laser-induced plasma on the surface of an aqueous solution. Using well-known equations describing the radiatif transfer and the evolution with the optical depth of the relative uncertainty on the measurement of line intensity and on the half width at half maximum, we have also deduced the uncertainty on the measurement of electron temperature and the electron density. An application to the diagnostics of this plasma has been realized, and we show the reliability of the spectroscopic measurements.