An underwater siting guide for use in international and territorial waters must be created to evaluate the consequences of the release of fission products from a nuclear reactor sited in the ocean. This paper is an initial attempt to develop the basic equations for such a guide. A very conservative fission-product-release inventory to illustrate undersea application is developed consisting of 100% of the soluble and 1% of the insoluble fission products or ∼66% of the gross fission-product inventory. The ocean is divided into four distinct zones for which current velocity profiles and characteristic diffusion parameters are established. Based upon a three-dimensional diffusion model incorporating shear effect and with the assumptions of no current variance, zero mean vertical current velocity, and depletion of the inventory by radiological decay only, equations are presented which describe the physical transport and dispersion of the radioisotopes. A computer program, SEADIF, is applicable to a person immersed in the water and is used to determine, for both contained and uncontained systems, the distance factors and the radioisotope concentrations as a function of time and position. Representative results for a 10 MW(th) system are given.