It is a complete (and dangerous) myth that CPR is not effective for fresh water drowning victims.
In fact, victims of saltwater near-drownings (who may or may not have recieved CPR or rescue breathing) are at risk of an additional complication, the so-called secondary drowning.
Secondary drowning occurs when enough salt from a near drowning is left in the lungs to create an osmotic gradient which draws water from the blood into the lungs, a condition known as pulmonary edema. The lungs fill with fluid and the patient effectively drowns – again.
This can complicate treatment for saltwater drowning victims, and is why it is important for anyone who has inhaled significant amounts of saltwater to be checked out at an ER even after they are rescued and just want to go home…
CPR (and rescue breathing) is incredibly important in any case of drowning where a victim has stopped breathing or their heart stoped beating.
It is admittedly often a desparate measure, with varying results which are dependent on many conditions.