The aim of this chapter is to assess the distribution of natural radionuclides (226Ra, 232Th, and 40 K) as well as anthropogenic radionuclides (137Cs) in the marine world. The determinations of these radionuclides were carried out using a high-purity germanium HPGe gamma-ray detector. Therefore, in this chapter, the patterns of radiation from natural and man-made radionuclides in marine environments worldwide were reported. This chapter covers the research results on radiation in the Mediterranean Sea, Caspian Sea, Red Sea, East China Sea, and Arctic Ocean.