The player's level of sunlight is represented by the letters SPF. To not get sunstroke, which can be caused once the sunlight meter is fully depleted you must either swim underwater or stay in a shade.

How do you get SPF in Stranded Deep?

The flask requires you to make a lashing and use a regular coconut, which has not been opened. After you have those two items, combine them in your inventory, and the Aloe Salva will make you immune to the effects of the sun for a small amount of time. You can just eat an aloe plant to help cool yourself off.

How long does sunscreen last in Stranded Deep?

It is used to immediately cure the effects of Sunstroke and protects the player from it for 12 in-game hours. It can be crafted by combining the Aloe Vera plant and a Coconut Flask.