Heat engine is a device used to convert heat into mechanical energy. First law of thermodynamics gives a qualitative statement for conversion of heat into work. But nature has imposed certain restrictions on conversion of heat into work. For this we require a device we called the engine and the engine requires a medium called the working substance.
For conversion of heat into work with the help of a heat engine, following conditions have to be met with:
(1) There should be a body at higher temperature T1 from which heat is extracted. It is called the source.
(2) Body of the engine containing working substance.
(3) There should be a body at lower temperature T2 to which heat can be rejected. This is called sink.
Here we can treat the warm surface water of the tropical ocean as a source and the surface of cooler water beneath as a sink of the heat engine. Therefore the heat engine which is operating between the warm surface water of a tropical ocean and the surface of cooler water beneath is a possible concept.
No, the idea is not practical, because it is very difficult to extract the energy from the heat engine which works in the deep sea