Navigation hazards are obstacles that can impede the safe passage of a vessel. These hazards can include SANDBARS, reefs, SHOALS, and SNAGS. SANDBARS are long, narrow ridges of sand found in shallow water, often near shorelines. Reefs are rocky outcroppings that rise from the ocean floor and can be hazardous to ships. SHOALS are similar to reefs, but are made up of sand or gravel instead of rock. SNAGS are submerged trees or other objects that can cause damage to a vessel. All of these navigation hazards can be dangerous to ships and should be avoided.