There is really no way to know how long any breed will live.
Golden Comets are bred for production, so for me, I would expect 2-3 years on average. But probably about the same for just about any hatchery bird.
I'm sorry for your loss.
Without a necropsy (either you perform one yourself or send the body to your state lab )
it's hard to know.
Now speculation on my part. A stomach full of water is called Ascites. In laying hens often Ascites accompanies reproductive issues like Egg Yolk...