(Now that's a lot of water!)
The warning itself, says that giving an over heated dog ice water is a bad thing for your pup and can cause fatal bloat (acute gastric dilatation-volvulus (GDV)) from the chilly water resource.
But this well meaning warning is nothing but a myth.
I've seen the warning refuted by various sources that ice water does not cause bloat. The actual warning that's being distributed by folks may have had two independent events occur, the ingestion of ice water and bloat, but there has been no concrete evidence that the two conditions are related.
There is a concern for giving a distressed dog too much water or they're drinking it too quickly. That's where bloat can come from... too much, too fast.
What we here at Vader's World suggest is something we do with most warnings we see online. First we look up the subject on SNOPES or HOAX-SLAYER. If we don't see it referenced there, then we'll toss it out there.
So don't fret about ice water and your pup.
Just temper how much or how fast you feed water to your pup!
Resources: snopes, hoax-slayer.