Mines done that a couple times. the first time I retrieved the rat from the water dish reheated it with blow drier and he struck/coiled and ate it. The second time I just let him go and he ate it fine half submerged.
Maybe in your case if the rat was fully submerged he just lost where it was at. With sent everywhere and the water tainted with blood he was just unable to pinpoint exactly where it was as it was hidden under all that blood and rat scented water.