Some brief searching ("cooking rice with seawater") indicates that at a minimum you can use seawater diluted with fresh water for cooking if your survival depends on it. Seawater is around 3.5% salt. For perspective, that is something on the order of a soup spoon of salt per soda can volume of water (12fl oz/355mL).
Undiluted seawater suitability seems to vary by food[1][2], my cursory search didn't provide any factual specifics for rice, oatmeal, and vegetables. Some comments indicate you can boil fish in straight seawater.
That's all without considering pollution and natural toxins from algae blooms. The sources I skimmed mentioned commercially sourced culinary "sea water", I would expect some amount of filtering and heat treatment.
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.
No, it's well known that humans cannot safely ingest the quantity of salt contained in sea water. In a normal situation with available fresh water it would not kill you, but if you were stuck at sea with no fresh water the 35g/L of salt you'd be consuming is way beyond the recommended upper limit of 5g/day.
I understand that pasta cooked in seawater is passable as most of the water (and salt) is discarded. Normally rice absorbs all the water, I don't know if you were clever and boiled it in lots then drained it if that would work.
Would take a lot of fuel though, usually propane for the USA, I think? In the UK many use Butane instead, or CampingGaz - in any case you don't really want to use it all up when you are out at sea.
