The purpose of this study was to clarify the relationship between kick frequency, toe vertical velocity, and inter-joint coordinative patterns at the lower limb joints during Underwater Undulatory Swimming (UUS). Eight male swimmers performed the 20 m UUS with maximal effort. Underwater motion-capture was used to obtain hip joint angle, knee joint angle, and ankle joint angle using Cardan angles. The inter-joint coordinative patterns of the hip-knee and knee-ankle was evaluated using coupling angles. Coupling angles were calculated from a vector adjoining two successive time points relative to the right horizontal at the angle-angle diagram. The inter-joint coordinative patterns were distinguished by their in-phase and anti-phase behaviours. As results, during the transition phase between down-kick and up-kick, the hip joint transitions first, followed by the knee joint. Such phases were identified as anti-phase, and a significant correlation between kick frequency and the percentage of anti-phase coordination between the hip and knee were observed (r = 0.73), whereas there was no significant correlation for downward toe maximum vertical velocity. These results highlight the importance of the anti-phase as a preparatory motion for continuous movement without interruption between kicking cycles. Additionally, there was a high correlation between the kick frequency and the percentage of the proximal dominant phase for coordination between the hip and knee (r = 0.74, p < 0.05), whereas no relationship was observed for the downward toe maximum vertical velocity.