It is kind of even more convoluted like that, actually. Indeed what you need is elements, but elements as the game implies them (let's call them subunits). Because if you go on and click on an element of a merchant ship, you will be scratching your head: it is made of 3 ships!
Thus, the units in the Atlantic and Mediterranean blockades are made of elements (subunits) and each element of ships. Fortunately, most other classes of ships are unique elements. In some rare instances a single ship portayed as a single element will be a unique unit. Look at the Antilles Fleet of the French for
an example.