Perhaps this is a bit british-centered.

In the fight of the Whale against the Elephant, British government and opinion boasted of innumerable raids against fishermen harbours (eg: La Ciotat, Cassis, Frioul islands), unnoticing that no major sea port was captured against French (Marseilles, Toulon, Leghorn). Britsih ruled the sea, and the shore, but only half a day travel into open land.

Napoleon had units all over Europe, not because of british threat, but to get effective control of territories against local populations, against powerfull neighbouring Turks, Austrians, Prussians, and, secondarily, to share the burden of his army with his satelites.
According to memoirs , French troop in Italy seem to have been more in Po valley and Alps, facing Austria, that on seashores, waiting for british invasion.

UK certainly dispersed its effort much more than French Empire : more than half of british army was fractioned all other the Earth, and so thinly that only one major theatre of operation at a time could be adressed, which was much deceiptive for british Allies. A good example is Walcheren expedition, which virtually ended any reinforcement for Wellington, and drove some of his best units away (see Oman). Whereas French Empire, with all its coastal batteries and seashore militias, could use the bulk of its force for decisive action in heart of Europa.

British forces in Sicily were as a threat as French fleet in Toulon was : a force in being, with a totally passive role. Aside from control of Sicily, so resented that Queen of Sicily contemplated some Vêpres Siciliennes, the only british expedition of significance was against eastern Spain, and ended in hurried re-embarkation. Minor fights in Ionian sea could have been organised from Malta.

And I don't see how conquest of Sicily, or even Malta, could have change the state of seapower in Mediterranea. Any expedition to Turkey or Egypt, with Royal Navy on the heel, and fierce muslim opposition, would have been non-sense after Trafalgar.