The rate of dissolution of CO2 from a liquid CO2 phase to an adjoining liquid water phase is known to be retarded significantly by the formation of a thin clathrate hydrate film spread over the interphase boundary. Much account is currently made of this "barrier" effect in the investigation of ocean disposal, or sequestration, of CO2, an option to mitigate the emission of CO2 into the atmosphere. However, the primary, dominant mechanism of the "barrier" effect provided by hydrate films is still unclear and sometimes delusively explained. This paper aims to discuss possible mechanisms of the effect and reasonably estimate the dominant one. The mechanism thus estimated is a reduction of the solubility of CO2 in liquid water in thermodynamically stable hydrate/liquid-water coexisting conditions at a temperature below T-h, the hydrate/liquid/liquid equilibrium temperature under a given pressure, from that in metastable (< T-h) or Stable (greater than or equal to T-h) hydrate-free conditions, which causes a smaller driving force for the diffusive, or convective, CO2 transfer from hydrate-film surfaces to the adjoining liquid water phase compared to that for the transfer from hydrate-free CO2/water interfaces. It is also suggested that liquid CO2 drops, freely buoying up in the sea, may also suffer an additional mechanism of retarding dissolution: a hydrate film covering each drop should suppress the tangential mobility of its surface, thereby causing a decrease in the surface-to-seawater mass transfer coefficient for CO2.