I would think very little of a service provider unable to convert their tips to their perfered currency.
Also bare in mind that on a Cruise ship there is a very high chance that the money will be saved up until they go home, and If that's to Euro-land, then there is little point in you giving them dollers for them to have to convert back to Euros.
Besides - why would a European, going on a European Cruise, in European Waters, tip using a currency from the other side of the globe. Besides, unless I'm mistaken a Euro is worth more than a doller, so a €5 tip is worth more than a $5 tip. Having that said a £5 tip would be better again - particularly if you're visiting Gibraltar.