Air bubbles in the sea water have smaller particle sizes and are less released from thw water than those in fresh water. Even if they reach the water surface, they tend to stay there as foams. This paper treats the phenomena of advection and diffusion of buoyant air bubbles in open channel flows by both Eulerian approach and Lagrangian one. Through the Eulerian method, the effects of hydraulic parameters to air bubble behaviour are examined. A Lagrangian model is also presented, whose simulation results are compared with Eulerian ones and their differences are discussed.