An architecture of underwater sensor networks and a two-tier routing protocol for cluster-based underwater heterogeneous sensor networks-TTCB protocol have been proposed in this paper. The architecture includes the upper sensor nodes, the lower sensor nodes, sink node on the sea surface and the base stations on the land. All sensor nodes form two heterogeneous clustering structure. According to the initial energy, the remaining energy of the node, the consumption energy of the previous cluster head, the average distance between the cluster heads and other factors, the TTCB protocol elects the first-level and the second-level cluster head nodes, forms a dynamic cluster head chain. The simulation results show that the TTCB protocol can reduce the average energy consumption of nodes effectively and prolong the survival time of the network, and is suitable for a large number of monitoring nodes in underwater environments.