This thesis proposes a novel routing protocol for underwater wireless sensor networks (UWSNS). An UWSN consists of a collection of sensor nodes deployed at different depths. Normally, sensor nodes use multi-hop paths to communicate with sink nodes positioned at pre-selected locations on the surface level. This kind of networks can be used in a variety of applications such as environmental monitoring and undersea exploration. Routing in underwater is an important and a challenging activity due to the nature of acoustic channels and to the harsh environment. The purpose of this research is to develop a new multipath grid-based geographical routing (MGGR) protocol for UWSNs. MGGR uses disjoint paths (paths from source to destination with no common intermediate nodes) to route data packets from sensor nodes to sink nodes. The proposed algorithm assumes that the monitored region is divided into logical 3D grids. It also assumes that the sensor nodes have identical capabilities and are equipped with localization services. The algorithm consists of three main components: (i) a gateway election algorithm; responsible for electing gateways based on their locations and remaining energy level, (ii) a mechanism for updating boring gateways' information; allowing sensor nodes to memorize gateways in local and neighboring cells, and (iii) a packet forwarding mechanism; in charge of constructing disjoint paths from source cells to destination cells, forwarding packets to the destination and dealing with holes (i.e, cells with no gateways) in the network. Routing is performed in a grid-by-grid manner via gateways that use the disjoint paths to relay data packets to the sink node, MGGR is characterized by two important features. First, only gateways can act as intermediate nodes for relaying data packets to destination. This helps in reducing the number of copies of each packet propagated in the network and non-gateways can save their energy. Second, as long as there is at least one node in a cell with remaining energy greater than a predefined threshold, there is a node that can act as a gateway in that cell, and because data packets are forwarded in a grid-by-grid manner via gateways, the path is considered alive as long as there exists a gateway in each grid cell passed by a route. The performance evaluation of MGGR protocol has been conducted via analytical modeling and simulation. The reachability analysis shows that the performance of the protocol improves as the network density increases and this agrees with grid-based routing in MANETs. The simulation is conducted using the Aqua-Sim simulator which is developed specifically for evaluating underwater sensor networks. Generally, results show that MGGR is an energy efficient protocol compared to Vector Based Forwarding (VBF) routing protocol in all simulation setups used in the study. This is mainly due to the fact that data packets are forwarded only by gateways which reduce the number of packets propagated in the network. MGGR can also maintain a good delivery ratio while it gives relatively higher delay compared to VBF. The higher delay incurred by the protocol is due to the long paths followed by the packets.