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 tries to stop him. He tries to stop another man from throwing money down it as well.
He hears that people throw money down a hole for fun, so he installs a wishing well.
Sandy wishes for a fancy telescope that works underwater.
Plankton wishes to be big enough to crush his enemies like "the vermin they are."