I havent been on the Med Cruise BUT I am from Europe and have done alot of travel by bus. It would only be the larger buses which can take over 50 passengers, think Greyhound type buses. But also it depends on the contractor and model of bus. Some do have a portaloo, some dont. I have been on buses with a portaloo and honestly I have never used it, it can get nasty very quickly, if you know what I mean. Remember the buses are sub contracted to local companies and depending on the amount of people on your excusion you may get a smaller vehicle.
Generally speaking, for journeys of approx 2 hours, you wont get a bathroom break. Over 2 hours, you may get a bathroom break at a highway rest stop.
Most tour sites will have bathroom facilities or there will be nearby bathroom facilities for you to use.
For travel heavy all day excursions, use the bathroom last thing before you leave the ship, dont expect a bathroom break until you get to your destination, and be very aware of your time management once you reach your destination.