Tuna in spring water is OK for cats....but only as a treat. 

Because it is not a complete food (probably doesn't contain taurine and other vitamins and minerals cats need) it shouldn't be given to the cat too often.

Have you tried mixing some of the human food tuna with a cat OK tuna wet food to get Buffy to accept something new?  This is how I usually introduce something new to my cat. 

It also works for dry food.  I bought something new for my cat....and she hated it.  So I started giving her the new dry food mixed with her usual dry food.  She got used to the new stuff and in the end was happy to eat it without mixing it with something else.

You've got to be more cunning than your cat is, to get any co-operation!