How deep is it? It is possible the water is getting too hot and lacking oxygen. What you have done so far is good and the hose will bring the temp down and what I would have suggested.
How big is the pond and are there other fish in it? If so, how many? Are they doing the same or just this one?
I gather the pond has no filtration or fountain, waterfall etc? A fountain, or similar, would help but make sure it does not spray the lily leaves as lilies do not like that.