Navigational hazards are obstacles that can impede the safe passage of ships and other vessels. Two common navigational hazards are reefs and SHOALS. Reefs are large, rocky outcroppings that can be found in shallow waters and can cause significant damage to a vessel if it runs aground. SHOALS are areas of shallow water, often caused by sandbars or sediment buildup, which can also cause a vessel to run aground if it is not navigated carefully.