This study examines the application of additional water cooling on the mechanical properties of AA7075-T651 alloy friction stir welded butt joints. Underwater joints were produced in a wide range of welding parameters and compared to conventional FSW joints in terms of microhardness and basic mechanical properties. The low-cycle fatigue properties of selected FSW and UWFSW joints were also compared. It was stated that regardless of the welding parameters used, UWFSW joints outperform conventional FSW joints of the AA7075-T651 alloy in terms of YS and UTS, with only slightly lower ductility. The use of machining coolant enables the production of UWFSW joints with higher load-carrying capacity and greater ductility compared to FSW. Under low-cycle fatigue conditions, UWFSW joints exhibit a greater tendency for cyclic hardening, lower plastic strain amplitude, and fewer cycles to failure.