Truly as advertised. I think that the cow has a lot of character, and because of this I wish it had something to do besides swim. On the other hand, I feel like you achieved the intent of something relaxing to watch, like a fish tank. I mean you don't really play with a fish. Also, to my initial point, I like the inclusion of the buttons to the left. They do add some feeling of interaction with the cow as it does it's own thing. Like it's thanking me for the thumbs up.
I'll also say, I stuck around to see what happened when it runs out of air, and it wasn't a slog. very calming. Although at the same time very sad. Peaceful Corpse Tank Sound doesn't have the same ring to it.
So to sum up I guess; I like the character of it, I wish it did just one more thing besides swim, (maybe come up for air at some point?) and I think this delivered nicely on the stated mood of the piece.