Can i brine chicken in seawater?
Some considerations: 

Typical chicken brines range from 5 - 10% salinity.

The sea averages 3.5% salinity (there are variables such as region, distance from estuaries, and weather that also impact ocean salinity).  Further, sea water is not just salt and water, but has other dissolved minerals, not to mention potential pollutants.  

Can you do it?  Sure, but what's your goal, and is the the way to achieve that goal?