The beauty of the Mediterranean is that it is a wholy different campaign to anything you are used to
In the Atlantic you are used to engage slow convoys (<12 knots) of fat targets with variable escorts in the open ocean, but the Mediterranean is essentially a place to go hunting warships, which run fast (>15 knots) and well escorted by fleet destroyers. You can eventually find merchants, but either they will not be worth a torpedo because of their small size, or else be well escorted. In any case, -as already said- it's warship what matters there
So I would recommend going there only when you master the technique of U-Boat attacks, and are ready for an additional challenge
