How do they power the fridge in the Going Merry?
Although nothing related to electricity/fridges is mentioned in text in the Wikia article on Going Merry, you can find the following image there (with a rough Japanese counterpart here):


  


Presumably, this Water-Drawing Machine is used to power the refrigerator. As pointed out in the comments, all you need is a rotary motion source to use electricity at all times.

Although, the crew stores most of their food in the Storage Room, as indicated by this image:


  


You may also note that there are 3 orange trees, taken from Bellamere's plantation, which are used as a source of emergency food: