This publication describes the work undertaken by Working Group 7, Assessment of Fate and Transport of Radionuclides Released in the Marine Environment of the IAEA’s Modelling and Data for Radiological Impact Assessments (MODARIA II) programme (2016–2019). The work carried out was a natural continuation of that completed during the first phase of the IAEA’s MODARIA programme (2012–2015) by Working Group 10, Modelling of Marine Dispersion and Transfer of Radionuclides Accidentally Released from Land Based Facilities. In MODARIA II, the degree of complexity of the models used was increased in order to effectively consider additional processes, specifically uptake by biota, and the spatiotemporal scales of the simulations were also expanded. General information on the simulation of radionuclides in the marine environment using dispersion and transport models was compiled. This is intended for scientists in Member States considering development or adoption of models for radiological assessment following incidents or emergencies involving releases of radionuclides into the marine environment.