It means that among all types of seafood ( fish & so) shrimp is the tastiest. Using fruit here is like a metaphor for the most delicious and tasty among the seafood.

i agree with Kanina Beh!

I'm sorry, but I do not fully agree with Cherry's answer... it does not have to be the best or tastiest. To be the "fruit" of something means to be a product of it. So here it merely means that shrimps are a product of the sea, or come from the sea. You could also say, "my success today is the fruit of my labour" ... meaning your present success is a direct product of your hard work.