After a mammal like we human are die, the bacteria who normally are present in the gut will get beyond the walls of the gut into the abdominal cavity, because the cells of the intestines are damaged by death. These bacteria will start the rotting process and produce gasses like methane, hydrogen sulfide, CO2, making the torso bloat, and by lowering the specific gravity of the body as a whole under that of water will cause the body to float.
Drowning victims often don't actually inhale any water. They effectively asphyxiate themselves as their airway reflexively won't open in the presence of water. Even if water is inhaled, you really need only a tiny amount to do you in. There is plenty of air left in the lungs to keep you afloat. However, if you're deep underwater when you die, or there is a strong current, you're body will stay underwater due to the weight of the water above you being greater than your buoyancy, or the current moving you faster than you can float back to the top. In this case, you'll stay down until gasses in your body are enough to float you back up.
From the above it can be very clearly observed that the person who dies of drowning cannot float in the water immediately after death.Thus the police story appears to be a concocted one.
Your case that she has been killed long back and thrown into the water may hold good and strong
As far as evidence you cannot have one beyond the police investigation into the case. Your evidence may not be supported by police statements if the police are hell bent to protect the accused.
