Naturally occurring radionuclides (226Ra, 232Th, 40 K, 210Po, 210Pb, etc.) and anthropogenic radionuclides (Pu isotopes 238Pu and 239/240Pu, 241Am, 137Cs, 129I, 90Sr, 14C, 3H, 134Cs, etc.) are sources of radiation in marine environments. Because of human activity, radioactive contamination of seas and oceans has varied in recent decades. Anthropogenic radioactive fallout from nuclear weapons testing, nuclear waste disposal sites, nuclear plant coastal discharge sites, accidents involving nuclear-powered ships, nuclear power plant accidents, etc., has significantly contributed to this contamination. A literature review revealed that regions of the Pacific and Atlantic Oceans (the Irish, Baltic, and Black Seas) have high radiation levels. The Irish Sea merges with the North Atlantic Ocean at its northern and southern termini. According to the literature, the Irish Sea is the most radioactively contaminated region in the world because Sellafield reprocessing units release approximately eight million liters of nuclear waste daily, poisoning sediments, saltwater, and marine life. Furthermore, radioactive materials have been found in the Pacific, including off the coast of Japan, due to the Fukushima accident. The Chernobyl nuclear accident is responsible for the high radioactivity in the Baltic Sea. In the past, investigations were performed for natural radionuclide monitoring in different regions of marine environments by some research groups. Several researchers have selected two radionuclides for investigations in aquatic environments—anthropogenic 137Cs and naturally occurring 210Po—from a radiological perspective. Human radiation exposure is caused mainly by radionuclides, which are prevalent in marine environments where humans consume contaminated marine biota. Radiation exposure poses a serious risk to human health since it can lead to malignant tumors, metabolic dysfunction, and genetic mutations. Therefore, a thorough understanding of ocean radionuclide levels is necessary to assess health. This chapter comprises case studies of high-radiation zones in the marine world and their potential causes.