Tidal energy was one of the renewable and sustainable energy forms under rapid development in the past decades. In tidal energy conversion, the hydraulic machinery is the key work component. However, the tip leakage vortex and cavitation have negative influence on the energy performance and operation stability of hydraulic machinery. In the present work, a C groove method is developed to improve the energy performance and suppress the tip leakage vortex and cavitation around a NACA0009 hydrofoil in tidal energy conversion. The optimization mechanism of the C groove on the energy improvement and vortex suppression is that the groove can suppress the primary tip leakage vortex (PTLV) by the groove jet impingement, and suppress the secondary tip leakage vortex (STLV) by the groove channel flow. The numerical simulations under non-cavitation and cavitation conditions are conducted to investigate the effect of C groove with different groove depths. The results demonstrate that the C groove can improve the lift coefficient by a maximum rise of 1.04% under 50% δ, and improve the lift-drag ratio by a maximum rise of 2.85% under 25% δ, respectively. The C groove most effectively reduces the TLV volume and swirling strength under 50% δ. Meanwhile, the C groove makes the vortex core shrink and far away from the foil surface, and most effectively suppresses the TLV cavitation under 50% δ. Therefore, the groove depth of 50% δ is recommended as the optimal depth on suppressing the TLV cavitation.