Underwater is a harsh and dynamic environ- ment. Its physical properties are unsteady in the space-time domain and that complicates the communication process inside the water. Moreover, underwater transmissions suffer from many attenuations and scattering sources due to several environmental noises such as shipping, turbulence, wave dynamics, and the background life of the marine organisms. Therefore, opportunistic and cooperative communications are necessary to compensate for the prompt changes in underwater environments. In this paper, we propose a new opportunistic cooperative transmission scheme for under- water networks that is adaptable for various environmental conditions. The proposed scheme employs the key physical variables of the seawater, e.g. temperature, salinity, density, and waves speed, to indicate the proper relaying scheme jointly with the appropriate modulation method and the transmitting power level that suit the current transmission situations. The paper also proposes a study that demonstrates the effects of these variables on the underwater speed of sound which is the most commonly used carrier for underwater communication and hence affects the transmitted signal intensity. The results show that the proposed opportunistic cooperative transmission scheme provides an efficient compromise between the overall network performance and the energy efficiency at the nodes.