Hi here are the 3 most common versions of this joke;
Waiter, what's this fly doing in my soup?
Um, looks to me to be backstroke, sir...
Waiter, there's a fly in my soup!
Be quiet sir, or all the other customers will be wanting one.
Waiter, there's a fly in my soup!
It's OK, sir, there's no extra charge!
All versions of this joke are an attempt by the waiter to stop the customer making a fuss in the restaurant about the quality of the soup that was served, so he makes a joke instead to take the customer's attention away from the soup.