Engine operating between two different temperatures in tropics

2018-04-10 10:29:07

In Zemansky and Ditman's Thermodynamics book, the exercise contains the following question.

In the tropics, the water near the surface is warmer than the deep water. Would an engine, operating between the two different levels, violate the second law? Why?

You can find it here on page 158. Please note that it's not a homework problem. I merely don't understand the intent of the authors in asking the question. Why do the authors think that this might give an impression of an apparent violation of the second law? Do they have perpetual motion machine of the second kind in mind?