There are desalination tools available, which remove the salt (NaCl) but keep most other elements in the water. Many travelers across the sea have used them so far and probably drunk several liters of desalinated seawater per day and have been fine. So, obviously, other minerals or substances ingested with seawater in reasonable amounts are not (very) toxic.
Usgs.gov
Drinking seawater at the river mouth could be hazardous, though (less salt but more industrial toxins).
Drinking seawater dehydrates you. This is because human kidneys have a limited capability to concentrate urine, which means for every gram of salt they excrete they also have to excrete a certain amount of water, which is greater than the amount of water in seawater you've drunk. In summary, for every liter of seawater you drink your kidneys will excrete at least 1.5 liters of urine, so your water balance will be minus 0.5 liters. Drinking seawater is never a god idea to overcome dehydration, because every little amount of seawater you drink will result in a negative water balance. This is explained a bit more here.
Dehydration is lethal when you lose more than about 10% of water from your body, which is 7 liters for a 70 kg man. Drinking 1 liter of seawater would result in "only" 0.5 liters of your body water, which is far from lethal (assuming you were well hydrated on the beginning).