In this research work, channel capacity of two-dimensional (2D) optical code division multiple access (OCDMA) system is evaluated in the underwater wireless optical communication channel. The various water categories which have been investigated for capacity evaluation are pure sea, clear ocean, and coastal water. Also, lognormal probability density function is considered for studying the impact of weak turbulence in the underwater channel. For unity channel capacity, the transmitted power should be greater than 15 dBm for all the water types. The channel capacity increases with rise in transmitted power and decreases with increment in number of users. The channel capacity is lowest in coastal water and highest in pure sea. In clear ocean, channel capacity is intermediate between coastal water and pure sea.