Untreated seawater in not safe for human comsumption due to the following reasons:
Seawater is overloaded with salt, which is not easily processed in large doses by the human body. The system can't metabolize the excess salt, so it pulls stored fluid out of the body's tissues and sends it into the urinary system to flush the excess salt from the blood stream, causing dehydration. A loss of 15 percent of stored water causes organs to shut down, eventually resulting in death.
Bacteria and micro-organisms in seawater can be fatal. Just a swallow or two can contain enough bacteria to cause infection in the blood stream or intestinal system, and result in death.
Seawater contains contaminants like crude oil, fuel and industrial waste that could be toxic to the human body if ingested.