New data is presented related to high-pressure underwater laser-induced breakdown spectroscopy measurements. LIBS calibration curves in high-pressure water are shown for elements that are relevant to hydrothermal vent fluid chemistry. The use of internal standards in high-pressure solutions is shown, and the use of hydrogen and oxygen, produced from decomposition of water by the LIBS plasma, as an internal standard for underwater LIBS measurements is demonstrated. The use of double-pulse LIBS for high pressure underwater measurements provides improved sensitivity but it is shown that the technique is limited by reduction in the size of the laser-induced vapor bubble with increased solution pressure.