v R,
Sorry about your cat. It's tough losing them. But be aware your cat was a "Tuna Junkie".
"A cat's sense of smell is 8-12X that of a human. Tuna oil to them is fresh cod liver oil to us, only they LIKE it- and it's addictive. Know how you feel when you enter a kitchen where a pie's baking in winter and coffee's brewing? Fish is a salivary gland kick starter for cats. If you feed your cat that same "˜smell' daily, it loses it's effect over time. It may be best to not abuse it, so that if you ever need to use it as an appetite enhancer, it still has that effect. To break a tuna junkie addiction -feed variety, add tuna juice to other meats, gradually decreasing until none is needed."
I realize you used the water and not Tuna oil but I think it's still not good on a regular basis.
Here is an excellent article about Tuna and cats
Having said all of this I did something similar. For the past 25 years I've had one inside cat ( though I once had two ). Among my first was Fat Cat ( who was skinny! ). I spoiled her terribly as I do all my inside cats. For about 6 months I had her on an exclusive diet of the best Tuna in oil and of course she loved it. Then I wondered if this was a good idea and I switched to Tuna in water. But when I went to the Vet, she said this was a very bad idea. Tuna once a week is fine but as a 7 day a week diet it is very bad as described in the article. AAMOF the Vet said that dry foods without fish is generally the best diet.
Of course I couldn't take Fat Cat off her Tuna "Cold Turkey" so I weaned her off slowly and eventually to a diet of solely dry food ( not fish based ), with an occasional wet meal ( not fish ) and treats of course. She lived a happy healthy life to the age of 17. Her successor, Little cat was on a similar diet and lived to 19. The current inside cat is Squeaky. I've gotten older and softer. Squeaky's primary diet is a premium IAMS dry food ( without fish ), But I give her a can of premium wet canned food ( not fish ) 3-4 times a week. I hope she reaches 20 years old.
You should enjoy this. I've posted before that my block loves cats. There are many on the block that belong to us all. Some neighbors have even gotten many of the outside cats fixed. And they never lack for a meal. AAMOF its gotten quite competitive. I used to put out decent quality dry food like Purina. Then others on the block started putting out premium dry foods like Science Diet and Iams. Soon some bastards started putting out canned foods. The cats looked on with amusement. It's good to be a cat on my block

Well, wouldn't you know it, some big timer ( could Vox have been in Texas? ) wanted to win. I think he has a lot of money and he's been putting out Sheba canned food. That stuff is expensive ( like $.79 a can ), and he puts out a lot. The cats love the competition.
I can't compete with 100s of cans of Sheba but I think I'm going to catch the SOB. I called my Vet and she said "Why not go to a butcher and get fresh meat ground up". I did and the cats love it ( they are true carnivores you know ). I'm waiting to see what the SOB does to one up me. The cats are smiling.