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 Peritonitis, cancer, Salpingitis or similar. Another cause of fluid in the abdomen would be organ dysfunction (which can accompany reproductive problems too - sort of like one thing starts to fail and the whole system breaks down.
It can be common to lose a few hens in a year to reproductive problems, especially if they are all the same age - sadly, hens are not long lived. Some respiratory diseases (if you have a history of one of those) also impacts the reproductive system and the long term results are seen a bit later in life.
If you still have the body and want to give a look, take photos and post them here, we will try to help you with what you see. Refrigerating and sending to your lab gives you the best diagnostics.