A group of university researchers and Technological Research Association of Mega-Float made continuous measurement of temperature and salinity of the sea water around Mega-Float model, moored off Oppama, from August 1996 till December 1998. The purpose of the measurement was to examine the impacts of Mega-Float on surrounding marine environment and it was found that the effects were very small. In the meantime, these observation data is very useful for the verification of the results of numerical simulations which is thought to be one of the useful tools for environmental impact assessment. So we calculated the current flow, water temperature and salinity in the summer of 1996 in Tokyo Bay. In numerical simulation, to compare the results of numerical simulation with observation data directly, we use the observed field data such as the amount of solar radiation, wind velocity and direction, etc. as boundary conditions. Additionally, the effects of boundary conditions on the results of numerical simulations were also examined. It was clarified that the variations of water temperature and salinity were predicted well quantitatively and the simulation results are affected by the boundary conditions such as extinction coefficient of sunlight and wind velocity.