No, they won't wash off immediately, fingerprints can be detected up to 6 weeks after being submerged, but most likely they'll be unrecognizable after 10 days. Which is still plenty of time to retrieve the knife from a body. Yes, from a body, because knife MOST LIKELY won't slip out of wound by itself, due to something called greater omentum, which is apron-like fat structure, which in case of stabbing abdomen, will wrap itself around object that punctured you, trying to hold to that object and create some sort of "seal" around the wound.
And in criminal cases, really, detectives aren't that closed minded, that they just look at one wound and say "well, that's what killed him". They REALLY depend on opinions from autopsy made by forensic pathologists.