We present experimental and computational results on the breakup of underwater buoyant oil jets and plumes at a wide range of Reynolds, Weber, and Richardson numbers and viscosity ratios. The results show three main jet breakup regimes: atomization, skirt-type, and pinch-off The threshold Weber number for the atomization regime is around 100, which varies slightly with the jet Eotvos number. Furthermore, it is demonstrated that the correlation proposed by Masutani and Adams as the boundary for the atomization regime applies to our broader data set too. The experimental and computational results both suggest that in a buoyancy-driven jet breakup occurs only when the jet is accelerated to a point where the local Richardson number, defined based on properties at breakup, becomes less than 0.4, in which case the local Weber number is above 10. The computational results reveal the mechanisms leading to formation of small droplets around the perimeter of energetic jets and umbrella-shaped jet separations at less energetic cases. The time-averaged lateral expansion of the simulated jets, representing four different conditions, is presented as a function of the height along the jet. The computational results were obtained by using a GPU-accelerated MPI parallel two-phase flow solver, which provides acceleration factors between 3 to 6, compared to running on CPUs only.