The detection capability of long-pulse laser-induced breakdown spectroscopy (LIBS) for the simultaneous analysis of submerged solids and bulk water has been investigated with a Cu target immersed in a CaCl₂ solution. It is shown that a short pulse leads to the formation of Fraunhofer-type absorption lines at shorter delays and severe self-reversal structures at longer delays for Cu. In contrast, a long ns pulse leads to high-intensity clear emission lines free from self-reversal structures. Especially, the signal enhancement for the spectral lines of Ca by the long-pulse irradiation is more significant than that of Cu, with the line intensity being increased by more than 10 times, even though the breakdown threshold of the liquid is not reached. Also, the use of a long pulse broadens the variety of detectable elements. We found that the atomic lines of H and O in water can be observed, and the relative standard deviation (RSD) of the LIBS signal is reduced by nearly 3 times by employing O as an internal standard. The obtained results suggest that the simultaneous detection of submerged solids and bulk water can be well achieved by using a long pulse with a low laser pulse energy, enabling long-pulse LIBS to be more advantageous in underwater in situ applications.