Answer :
Salt water is a very hypertonic medium. The osmosis gradient is the difference in concentration between two mediums. In this case, the salt concentration outside of the blood cells (body) was much higher than the one inside (they were in a hypertonic medium). This differences promotes the exit of water from the blood cells to the extracellular medium. The main thing we could expect to see in a blood sample of someone that drowned in salt water is that his/her erythrocytes were shrinked because all the water exited the cells.