@slats.
I am not sure that there would be much value in determining carboxyhaemoglobin or lactate levels in blood from bodies immersed in hypertonic salt water...Likely the hypertonicity would significantly alter biochemistry.
Examination of lungs and detecting diatoms could give an indication of whether people were alive or deceased prior to immersion, although this is still not 100% conclusive at autopsy.
(apologies for graphic content)