ID
Yes, by all means tip in USD. The driver will have no trouble changing into Euros. Local Camorra, Ndrangheta or similar will look after him. They have an unsatiable appetite for USD. (applies throughout Italy) May already be paying them a fee for security protection.
The water taxi he owns will be the equivalent cost of a Bentley or Roller in London. The medallion, Dario references, in all probability, has a value far greater than the house and 401k and other assets of most posters on these blogs. The medallions are tightly controlled. If it makes one feel good to tip in addition to paying circa $120USD per hour, do it.
Two of my favourite airlines, Emirates and Qatar, use Marco Polo airport. It is a delightful airport that suits my arrival and departure times.
Regards Ron