Don't forget that animals will often bury stuff that smells bad out of instinct. At one point, we had four cats, and I'd find little piles of poop on the floor covered up by a Kleenex or a piece of notebook paper or something. We had one cat who simply could not grasp the concept of a litter box and how to use it, it was the kids' job to clean up after him, and I was pissed because I thought that they were literally covering it up rather than dealing with it. Then one day, he dropped a pile right on the floor while I was watching TV, and I thought "Damn if I'm gonna clean that up." I didn't even want the stupid thing in the first place. Then one of the other cats came by, sniffed at it, and kinda scratched at the carpet (except she has no claws) to try and "bury" it. Obviously it didn't work, so she grabbed a Kleenex out of the wastebasket and covered it up! Mystery solved.
Anyway, it seems to me that since dogs don't eat raw fish, the fish is just something smelly and the dog is trying to "bury" it with the water.