We study the problem of distributed time synchronization of underwater sensor networks, which is a quite challenging issue under such a dynamic scenario. Due to the high and varying transmission delay caused by the mobile network nodes, the existing works cannot achieve a desirable accuracy of synchronization. To overcome this problem, a mobility built-in model is firstly explored for the time synchronization in consideration of the effect of mobility and transmission delay. Then, a time synchronization equation is formulated according to the proposed model. Based on the equation, a mobility built-in model based time synchronization algorithm, namely MM-sync, is designed. The experiment results show that MM-sync cannot only reduce the consumption of energy, but also achieve a higher accuracy than state-of-art solutions.