To keep your hydration levels up you need moisture.  You can use either water or coconuts for this.  You get water by using a desalinator on a beach square, and get coconuts by going to a coconut tree and picking coconuts (in Move menu).  Once you are carrying coconuts or water, you may consume them by selecting the appropriate option from the Consume menu.

To keep your nourishment levels up you need  fish or bananas.  You get fish by using a rod on a beach square, and get bananas by going to a banana tree and picking bananas (in Move menu).  Once you are carrying fish or bananas, you may consume them by selecting the appropriate option from the Consume menu.