Not sure if this happens on a Merc, but if you look at the lens there should be a wedge/pie slice shaped bit to the lens, about 6" long and 1" tapering to 3" (very rough dimensions). This is the bit the dips the beam, so this is the bit you need to cover. Normal tape is not good enough, as the light still shows through, but I think you can get a kit from Halfords (or similar) that has silvered plastic that you cut out and stick on. You can pre cut this and then stick it on on the ferry. Have a nice trip.