What causes the immediate sharp sting felt upon entering seawater?

Rapid osmotic shift as water is drawn out of skin cells due to the hypertonic solution

The sharp, immediate stinging sensation experienced upon contact with the ocean is primarily a consequence of osmotic shock. Seawater is classified as a hypertonic solution, meaning it possesses a significantly higher concentration of dissolved solutes, mainly salt, compared to the internal fluid within human skin cells. When the skin, particularly if it is dry or has minor abrasions, meets this highly saline external environment, the physical laws of osmosis dictate that water molecules naturally move from the area of lower solute concentration (inside the cells) to the area of higher solute concentration (the seawater). This rapid withdrawal of water causes the outermost skin cells to temporarily shrink or collapse, which in turn stimulates the underlying nerve endings, registering the distinct sensation perceived as stinging or burning.