At one point I wondered how seawater killed you since there isn't anything (directly) toxic in it.

Your kidneys excrete the excess salt you ingest. The problem is, sea water has higher salinity than the kidneys can excrete so your kidneys have to excrete more water than you ingested of sea water and you actually die of dehydration.

This is why it is so horrible: the more sea water you drink, the more dehydrated you become, the thirstier you get, and the faster you die.

Ref: https://oceanservice.noaa.gov/facts/drinksw.html

See also salting slugs: http://www.slugoff.co.uk/slug-facts/salt

According to the book "Salt, Fat, Acid, Heat" the optimal salinity for pasta water is about 2%. So 1 part sea water, 1 part fresh water - roughly.