This work explores the applicability and effectiveness of electrical resistivity tomography in mapping archaeological relics in the shallow marine environment. The approach consists of a methodology based on numerical simulation models validated with comparison to field data. Numerical modelling includes the testing of different electrode arrays suitable for multi-channel resistivity instruments (dipole-dipole, pole-dipole, and gradient). The electrodes are placed at fixed positions either floating on the sea surface or submerged at the bottom of the sea. Additional tests are made concerning the resolving capabilities of electrical resistivity tomography with various seawater depths and target characteristics (dimensions and burial depth of the targets). Although valid a priori information, in terms of water resistivity and thickness, can be useful for constraining the inversion, it should be used judiciously to prevent erroneous information leading to misleading results. Finally, an application of the method at a field site is presented not only for verifying the theoretical results but also at the same time for proposing techniques to overcome problems that can occur due to the special environment. Numerical and field electrical resistivity tomography results indicated the utility of the method in reconstructing off-shore cultural features, demonstrating at the same time its applicability to be integrated in wider archaeological projects.