An apparatus for detecting the presence of gaseous analytes in water includes a submersible housing enclosing a water proof sensing chamber having a first wall made of a material transparent to UV light and a second wall formed by a gas permeable membrane for admitting into the sensing chamber gas dissolved in water in which the housing is immersed during operation, a first and a second spaced sensing electrodes disposed within the sensing chamber, and a circuit connected to the sensing electrodes for measuring a current flowing through the sensing electrodes on account of ionisation of a gas in the sensing chamber by UV light. A third electrode is positioned in the sensing chamber, and the circuit is operative to apply a voltage across the third electrode and the first sensing electrode, the voltage being of sufficient magnitude to reduce condensation within the sensing chamber by hydrolysing any droplet of condensation in electrical contact with the first and third electrodes.