Groundwater vertical well cleaning technology, utilizing the shock wave generated by high-voltage underwater pulsed spark discharge, presents a promising application. In this study, we conducted experiments with a specially constructed pressure chamber to investigate shock wave conditions under varying ambient pressure up to 20 atm and cable length up to 160 m. A significant decrease in shock wave intensity is observed for longer cable and higher ambient pressure due to increased transmission loss in the cable and energy consumption during the pre-breakdown process. We also demonstrate that the reduced shock wave intensity can be significantly improved by applying an additional preconditioning technique, which involves the generation of microbubbles through electrolysis of water. The experiments carried out under the ambient pressure of 20 atm shows ~3.5 times increase in the shock wave intensity when the appropriate preconditioning is applied.