Wash the fin on your back, part of the deal(shark)

The story:

I always heard it this way, and since I was confusing "loan shark" with "deal shark," I thought it was an actual term, so one day I was trying to figure out what a deal shark was, and why they were telling him to wash his fin. It suddenly hit me that the lyric wasn't "wash," but "was." Then I felt stupid. But I love the mental image of a shark with some soap, scrubbing...