Typically yes, as they tender to the different port at Santorini. That being said, not all excursions start at approximately the same time. Many cruise lines offer a number of tours with staggered departure times. So, if your ship arrives at 8am, most likely the longer tours (say 6-8 hours) will typically depart first. short tours (2-4) hours may not depart until later, or even have different sections that might leave at two or more different times during the visit.