Seawater is entirely non-drinkable because of many reasons.
80% percent of seawater contains salt, which in turn is terrible for your health if you drink it. This is because when the water passes through your digestion system, there are little nutrients that can be extracted, and the small amount of water there is would be dried out. Second of all, the salt would make you thirstier, worsening your condition of thirst and probably motivating you to drink more. This can lead to death. Furthermore, dead fish, dissolved plastics, bird droppings, sand, bits of rock, and seaweed are inside seawater, and these, obviously, contain masses of bacteria that can make you very ill if taken in large amounts. So don’t drink seawater.