Swimming in the ocean is substantially more difficult than swimming in a pool or most other places because you must fight with waves, tidal changes, the slope of the beach, and powerful currents that may exist even when the surface seems calm. Even when the water is flat, there can be large forces acting on a swimmer that can cause him or her to drown. For example, if a person is drowning and someone throws a rock at them, it might be enough to hurt but not kill them. However, if the rock was heavy enough, it could compress the chest cavity until it stopped their heart. This shows that even harmless objects can have deadly effects if placed in the right position. Also, ocean beaches are generally not as clean as pools. If you fall over in the sand and don't get up, people will walk by without noticing you because there's nothing unusual about the situation. But if you were in water where you couldn't stand up, such as during a storm, then others would come to your aid. Finally, there are animals in the ocean that can cause harm even when they appear friendly. For example, sharks are known for their viciousness, but they will only attack if threatened. So, if you were swimming in the ocean and a shark approached, you would be in danger regardless of how friendly it looked.