To decommission the Fukushima nuclear power plant after the accident caused by a tsunami in 2011, characterization of the fuel debris is required. The precise location and radiological composition of the fuel debris are currently unknown, and the area is submerged making it difficult to investigate the primary containment vessel. An integrated system that includes both radiation detectors and sonar will allow the full localization and characterization of the fuel debris. This paper describes research completed toward the development of a complete system, on-board a low-cost, small form-factor, submersible remotely operated vehicle. A cerium bromide (CeBr₃) scintillator detector for dose-rate monitoring and gamma-ray spectrometry has been integrated and validated experimentally with a ¹³⁷Cs source, both in the laboratory and while submerged. The addition of an Imagenex 831L sonar has enabled technical demonstrations to take place at the National Maritime Research Institute's facility in Japan, where the system was able to characterize the shape and size of synthetic core debris. The combination of geometrical and radiological measurements allows the real-time localization of fuel debris and isotope identification, leading to an invaluable source of information to the workers at Fukushima that will enable increased efficiency and reduce risk during the decommissioning of the site.