Assuming it’s running in a bundle with other cables, ampacity of 6AWG cable (assuming you’re using boat cable with a 105C insulation rating) is 100A. I’d recommend using a fuse that’s significantly higher than the breaker you’re using but lower than the wire ampacity, such as 80A. That way, the long wiring run to the helm will be adequately protected from short circuit damage, and if the circuit is temporarily overloaded, the resettable breaker will trip before the fuse blows, making it easier to restore system function.