In this paper, we investigate the transmission mechanism and eddy-current loss of the contact-less power transmission (CPT) system in seawater environment. Contact-less power transfer could be achieved in the three following ways: magnetic coupling, magnetic resonance coupling, and microwave radiation. When the primary and secondary coils are in resonance, a channel of low resistance in the magnetic resonance coupling system is formed. Therefore, it is used for medium-distance power transmission and it has less restrictions on orientation, which means that it has wide applications in many scenarios. Moreover, contact-less power transfer is safer and more concealed than traditional plug power supply, especially in underwater vehicles. Firstly, the mathematical model based on the mutual inductance model is proposed for the CPT system in the air, then the frequency analysis of the CPT model as well as theoretical explanation of the splitting phenomenon is conducted, after that we consider the seawater effect on the mutual inductance coefficient. Secondly, we build a mathematical model of the eddy-current loss in seawater circumstance according to the Maxwell's equations, where we introduce an average magnetic induction in cross section, then derive an approximate formula through Taylor expansion, and analyze the relations between eddy-current loss and the physical parameters including coil radius, resonance frequency, transmission distance, and magnetic induction. According to the theoretical results, we optimize these physical parameters and then design a 754 kHz CPT system, thereafter we validate the CPT system both in the air and in seawater and find the difference between these two circumstances, and verify the relations between eddy-current loss and the physical parameters which are proposed in our theory. It can be learned from the experiment that when transmission distance is 50 mm and transmission power is 100 W in the air, the transmission efficiency is over 80%, and when transmission distance is 50 mm and transmission power is 100 W in seawater, the transmission efficiency is over 67%. Apparently, our magnetic-resonance-coupling-based CPT system has potentials serving as an underwater vehicle.