A watercraft has a hull, an engine compartment, an engine disposed and a jet pump. A gate is pivotable relative to the jet pump about a gate axis between an up and a down position. A gate actuator assembly includes an actuator housing fixed to the hull, and an actuator arm movable along an actuation axis with respect thereto. The actuator arm is operatively connected to the gate for pivoting the gate about the gate axis. A sealing member encloses a portion of the actuator arm between the actuator housing and the gate. The sealing member extends between a first end fixed with respect to the actuator housing and a second end fixed with respect to the actuator arm, forming a variable volume outer chamber to prevent entry of fluid therein. A passage fluidly communicating with the outer chamber, extends through the hull into the engine compartment.