We just took an Oceania excursion in Santorini few days ago - yes, you're taken to Athinios port and are bused to your destination from there. That was the easy part, though the bus ride up the mountain was a bit extreme lol The tour finished in Fira and were provided with vouchers for the cable car. There were 4 cruise ships in town that day, and it took us exactly 50 min to board the cable car. Honestly, it wasn't so bad, and the line moved quickly, as they board 36 people at a time. However, Oia and Fira were so crowded, it took all the enjoyment out of it - personally I wouldn't even bother leaving the ship with 8 others in town that day.