A novel approach, named guided conversion enhancement, has been established to improve the laser-induced breakdown spectroscopy (LIBS) sensitivity of cation detection in water. Two processes were involved in this approach: the main part was replacement reaction that converted the cations in water to solid granules on the surface of an immersed metallic sheet; the other was electric assistance that increased local cation concentration and strengthened the reaction. With the aid of replacement reaction and an electric field, a detection limit of $16\mathrm{ppb}$ was achieved for copper cation ( ${\mathrm{Cu}}^{2+}$ ) detection in a water solution of $\mathrm{Cu}{\mathrm{SO}}_4$ . The obtained results suggest that this approach has significant potential to be developed as an effective method for underwater cation detection.