This experimental study examines the mechanisms causing cavitation breakdown in an axial waterjet pump. The database includes performance curves, images of cavitation, measured changes to endwall pressure as the blade passes, as well as velocity and pressure distributions inside the blade passage, the latter estimated using Bernoulli’s Equation in the rotor reference frame. They show that cavitation breakdown is associated with a rapid expansion of the attached cavitation on the blade suction side (SS) into the blade overlap region, blocking part of the entrance to this passage, increasing the velocity and reducing the pressure along the pressure side (PS) of the blade. Initially, expansion of the SS cavitation compensates for the reduced PS pressure, resulting in a slight increase in performance. Further reduction of the inlet pressure causes a rapid decrease in performance as the SS pressure remains at the vapor pressure, while the PS pressure keeps on decreasing. In addition, during the breakdown, entrainment of the cloud cavitation by the tip leakage vortex generates the previously observed perpendicular cavitating vortices (PCVs) that extend across the passage and reduce the through-flow area in the tip region. Tests have been repeated after installing circumferential casing grooves aimed at manipulating the tip leakage flow and reduce the formation of PCVs. These grooves indeed reduce the tip region blockage during early phases. However, they have a small effect on the performance degradation by cavitation breakdown, presumably owing to their limited effect on the attached SS cavitation and tip region cloud cavitation.