The lifetime assessment experimental platform described in this report is specifically designed for testing the low-speed main shaft seal for Verdant Power's (Verdant Power Inc., New York, New York) fifth-generation (Gen5) underwater power generation turbine. This project is jointly sponsored by the New York State Energy Research and Development Authority and the U.S. Department of Energy as part of the Roosevelt Island Tidal Energy Project. The seal test stand was designed and built by Manufacturing Resources Inc. (MRI, Cleveland, Ohio) and originally put into operation by Garlock Sealing Technologies, Palmyra, New York for validation and initial testing in 2015. This report verified the system performance, including the overall efficiency of the tidal energy converter. The opportunity for the National Renewable Energy Laboratory (NREL) to conduct accelerated lifetime testing of the main shaft seal using the modified MRI/Garlock test stand provided a unique effort to demonstrate wear and tear and predict the 5-year performance of an underwater component of the 5m Gen5 turbine. This report briefly discusses the observations and results from accelerated lifetime testing performed by the NREL on the main shaft seal for the Verdant Power Gen5 underwater tidal energy converter turbine, which successfully performed at the Roosevelt Island Tidal Energy project in 2020-2021. Based on the results, it is recommended that follow-on testing be conducted through NREL's Testing Expertise and Access for Marine Energy Research (TEAMER) program to rectify protocol and assembly issues to further evaluate the SI of this component. This project is part of the TEAMER RFTS 2 (request for technical support) program.