Hello Scargirl. I've given all my dogs a bit of the water when I strain the tuna for myself. It's not a whole lot and the dogs really like it. As a once in a while thing it shouldn't hurt your dog.
If there's added salt of any kind I wouldn't give it tbh. I don't know enough about dog nutrition to say how much salt they should be getting per day or if that's too much, maybe someone else might? Better safe than sorry though!