Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Casey became lethargic, drank a lot of water, vomited green/yellow material and passed away. My condolences on your loss. I hope I can give you some peace so you can focus on getting yourself better rather than dwelling on his passing.
Dogs that have a gastrointestinal blockage or very abnormal intestinal motility often vomit green material as that is digested bile, bile that sits in the stomach for a while. Bile only refluxes back into the stomach when gastrointestinal motility is abnormal, such as with a blockage or poor gastrointestinal motility due to primary intestinal or metabolic disease.
Vomiting can be related to chronic pancreatitis, esophageal reflux, a dietary allergy or sensitivity, toxin exposure, a viral or bacterial infection, inflammatory bowel disease, internal organ failure, a full or partial gastrointestinal obstruction or an abdominal tumor compressing the gastrointestinal tract or infiltrative intestinal cancer such as lymphoma.
Was he a fellow that ate things outdoors or chewed on things such that he could get into something you don't know about or eat foreign material that could lead to a blockage? If so a blockage that caused an intestinal rupture, peritonitis and sepsis could have led to his death.
An abdominal mass that was blocking his intestinal tract and ruptured could also cause sudden death. Unfortunately Goldens as a breed are very prone to cancers, especially lymphoma and hemangiosarcoma. Either of these tumors could have been responsible for his death.
Finally toxin exposure and organ failure is possible, but much less likely given how quickly things occurred. Bromethalin is a rodenticide that causes brain swelling and neurologic damage and could cause sudden death, although usually we see incoordination and seizures too.
Given your fellow's age and breed I would be highly suspicious of an abdominal tumor that grew big enough to cause a gastrointestinal blockage and it either bled or ruptured leading to your fellow's demise.
If things happened as quickly as they seemed to it is likely your fellow didn't suffer much.
I am so sorry that the New Year has started so horribly for you. Please take care and get well.