Two Q-switched Nd∶YAG lasers operating at 1064 nm are used and combined in orthogonal beam geometry to perform underwater single and double pulse laser-induced breakdown spectroscopy (SP-LIBS and DP-LIBS) experiments in barium chloride (BaCl2) solution. The influences of key experimental parameters on the spectral characteristics and signal enhancement of underwater SP-LIBS and DP-LIBS are investigated systematically. There is a time window from approximately 500 ns to 900 ns in underwater DP-LIBS, where signal-to-noise ratios are very high. The time window favors the selection of delay time for spectral acquisition and analysis. By properly setting the relative position of two laser foci and using optimized laser energy, a 20-time enhancement of the Ba II 455.4 nm line intensity of DP-LIBS over the maximal intensity of SP-LIBS is obtained. The spectral line of DP-LIBS is narrower and last longer than that of SP-LIBS. In addition, dependence of spectral line intensity of DP-LIBS on the second laser energy is observed, which demonstrates an exponential increase. This is quite different to the linear growth observed in collinear DP-LIBS experiment on the liquid surface, and the difference in growth trend may be related to the gaseous environment in which the second laser induced plasma is produced and confined. Finally, the detection capability of underwater SP-LIBS and DP-LIBS is compared. The results show that the detection sensitivity of DP-LIBS is improved by a factor of 37, and the limit of detection of Ba in bulk water is reduced from 31.35×10 -6 in SP-LIBS to 1.78×10 -6 in DP-LIBS.