A water-proof electrochemical sensor for in situ analysis and real-time monitoring of chemical and biological compounds in a liquid analyte, such as a water column, or a water column/sediment interface, or sediment. The water-proof electrochemical sensor assembly comprises an electrode body that is resistant to high temperatures and pressure and includes a semi-hard o-ring or collar. A resilient elastomeric boot, that has an internal groove which is adapted to form a tight pressure-fit with the collar is installable on the electrode body. One end of the boot has an opening adapted to form an interference fit with a pin contact for electrical connections with external equipment and the other end of the boot has an opening adapted to form an interference fit with the electrode body whereby a water-proof seal is formed.