Opportunistic routing is appropriate for the Internet of Underwater Things (IoUT) with low link quality and unstable networks because it can choose candidate forwarders and leverage the collaborative forwarding of several nodes to increase the dependability of data transfer. This paper focuses on the design of an Opportunity Routing protocol based on Density Peaks Clustering (ORDP), which mainly consists of three phases: network clustering, cluster head election, and opportunistic data transmission. First, the Density Peaks Clustering (DPC) is employed to cluster the network. Secondly, for cluster head election, remaining energy, packet delivery ratio, and node distance are combined using the entropy weight-TOPSIS approach. Then, with the help of cluster head nodes in the forwarding set, an opportunity transmission mechanism is designed. Simulation results demonstrate that ORDP perform better than related protocols in terms of energy usage, average transmission latency, and packet delivery ratio.