Way too much sodium in seawater, like over 10g per liter. You could dilute seawater, but I think you’d have to add potassium.
Edit: Plus it tastes bad. Maybe not if you got it from the open ocean, but it’s going to taste heavily of funky decaying sea life
I imagine you could drink 8 or 12oz no problem. A lot of the pollutants and gross stuff is so diluted it doesn’t matter. And think about the germs in our day to day life.
I’ll refer you to the George Carlin bit on his immune system from swimming in the East River as a kid.