The generation of plasma in underwater gas bubbles offers the potential to produce large volume plasma in water while minimizing electrode erosion. Such attributes are desirable for the design of plasma-based water purification systems. In this work, gas bubbles of diameter 0.4-0.7 mm were trapped in the node of a 26.4 kHz underwater acoustic standing wave and pulsed with voltages in the range 10-14 kV. Plasma formation in trapped, isolated bubbles was observed to occur through two separate pathways: (1) plasma generated in the bubble through impact by a liquid streamer and (2) plasma generated in the bubble due solely to the applied electric field. The former case demonstrates the mechanism of so-called streamer hopping in which the discharge transitions from a water streamer to a gaseous surface streamer. Perturbations of the bubble's fluid boundary due to the streamer are also discussed.