I have been smoking meat recently, and I found out that in Maine people brine lobster in seawater. Can I do the same in Florida with a chicken?
Consider boiling down your seawater first. Because
Brining is better in solutions saltier than seawater.
Seawater has lots of microbes in it that might like raw chicken.
If you boil it down, do not do it in a cast iron pan. I learned that the hard way.
Brining a chicken is not superior to rubbing a chicken with salt and letting it sit. I think rubbing with salt is less messy / fussy.
You can get sea salt which is tested to be delicious because they want you to buy more. Sea salt I have made myself is sometimes bitter or funky, and that is not including the cast iron pan blood-salt.
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?