What is the most useful language to speak if living in the Mediterranean?

The most useful would be English. The Mediterranean spans several countries, so it depends on where you are living. The most common languages along the Mediterranean are:

1. Greek
   
2. Italian
   
3. Turkish
4. Arabic
   
5. Hebrew
   

None of these languages are a lingua franca, and will only be useful in the regions they are spoken. If you're looking for a language that will work all over the Mediterranean, your best bet is English.