Snipers were a standard part of allied infantry organisation, and no doubt snipers were present at the landings. However, for a sniper to be effective they need to be able to see a specific target, and be able to shoot accurately enough to hit it. Seeing a machine-gunner, even if one could see where the machine gun was would be very difficult, especially with all the smoke and dust from artillery and air strikes around the potential targets. One must also bear in mind that ships and landing craft, even on a calm sea are constantly moving in all axes, even if notionally stationary. Add the bow waves of passing landing craft, landing ships, destroyers, fire support vessels etc, and a sniper, even if he could see a target would have virtually no chance of hitting it. If you look at this film clip, from about 5 minutes in, you'll see that no sniper would have been able to hit anything from a vessel. https://www.youtube.com/watch?v=QE2f4pbjJv8