The surface energy parameters of the invasive aquatic weed, Hydrilla vertcillata, were determined using contact angle measurements using two different methods. The abaxial and adaxial surfaces of the leaves and stem were characterized for the weed while submerged in water using captive air and octane bubbles. For comparison, the adaxial surface of air-dried leaves was characterized using sessile drops of three liquids. The results of captive bubble experiments estimate the surface energy parameters to be similar for each of the plant components with a surface free energy of ~45 dyn/cm with polar and dispersive contributions approximately equal. Air-dried leaf surfaces had an estimated surface free energy of ~47 dyn/cm with a dispersive component of 38 dyn/cm and a polar component of ~9 dyn/cm. These results highlight the need to take into account the hydration status of these types of surfaces when analyzing their surface energy components. This report is the first to characterize the surfaces of this important weed and of submerged plant components in general.