Although its not stamped you never know. Acid test it. Weigh the chain. Measure the length and then the width with calipers. Then see what similar chains for sale weigh. If you're in the ballpark congrats.
Every big chain I've ever found has been marked. SS will show low #s like gold. I'm surprised you haven't done a heavy scratch/acid test yet. Looks a bit shiny. Smaller chains I've found in the ocean often are broken , lost clasps , etc. The big chains never break , they fall off going back over the head intact , usually in big surf , diving , etc. GL on it being real for you.
Measure the Specific gravity and determine if really gold. I found 1 a few yrs ago, and found out it was 'Tin". The chain was even marked 14K and really looked, well real. Good luck.
Have you ever confirmed the validly of the acid test??? Acid is usually good enough for me but something this big needs further testing! Congrats on a truly golden recovery!!