It looks like a regular stone well and the top of it is made with straw. When Mr. Krabs sees a child throwing a quarter he was after in a well. He tried to stop another man from throwing money down that well, he heard that people throw money down a hole for fun, so he installed a wishing well.
Sandy wished for a fancy telescope that works underwater.
Plankton wished to be big enough to crush his enemies like "the vermin they are."