Abstract A dense and relatively cold plasma is obtained, by a single laser breakdown in underwater medium. For the first time, the Balmer H α line is observed in such conditions. The spectrum is studied with a spatial and time resolved spectroscopic method. Traces of Ca and K are added to the water sample for the temperature and electron density measurements. Values of N e as high as 6.2×10 18 cm 3 and temperature lower than 10 4 K are reached simultaneously. Temperature and electron density are varied over a large range by varying the laser energy. The stability and the radial homogeneity of the plasma are studied. Abel inversion is used to estimate these radial inhomogeneity. Observed line profiles are unfolded by a new method. The theoretical H α widths, deduced from N e and T measured with K and Ca traces, are compared with the experimental Balmer H α line widths. When the electron density grows, increasing discrepancies between them are observed. They are related to non-ideal plasma behaviour due to density correlation effects.