The real-time monitoring of marine radioactivity is essential for rapid response to radiological events, particularly in Korean coastal waters influenced by radionuclide transport from the Fukushima region. Conventional sampling-based analyses provide high accuracy but lack temporal continuity and broad spatial coverage. To overcome these limitations, in this study, we developed and evaluated a series of in situ underwater gamma-ray detection systems [Monitoring of Ambient Radiation of KAERI-Underwater (MARK-U) series], each optimized for seabed monitoring, dissolved radionuclide measurement, on-site sample analysis, and maritime mobile surveys. The systems employ NaI(Tl) and CeBr3 scintillation sensors integrated with pressure-resistant housings, embedded electronics, and real-time data communication technologies for underwater radiation sensing. Detection efficiencies of the sensors were calculated using Monte Carlo N-Particle (MCNP) simulations and verified through field experiments. The results demonstrate that the MARK-U systems provide stable, in situ gamma-ray spectrometry with significantly improved temporal resolution and operational efficiency over conventional methods. These integrated underwater systems support enhanced marine radiological surveillance and rapid-response capability for future environmental monitoring and emergency situations.