I'm sure this question is a cliche but why do our fingers and toes get wrinkled when we stay in water for a long time. It can't be osmosis because the water potential of water is not more negative than that of the cells of the body.

Probably osmosis moving water into the cells.

 

If the cells under the skin expand, the epidermal layer will become more convoluted.

The dermis layer of the skin loses protective waterproofing oils (wipe your hand on a mirror to see the oil from your hand) when it's soaked for too long. Once the layer is gone, the difusion gradient ensures that water enters the cells, making them turgid, so the skin surface expands somewhat. The expanded surface area causes the wrinkles.