Did Santorini on Breakaway the other week (july 18thish) and it was a zoo since our ship was coming in at 1pm. There are water taxi's that'll take you to other parts of the island but getting back is a pain. There should be no reason why any cruise line that has a ship greater than 2k pax should port here. my brother and i ended up walking up the steps (I a 250 pound guy), and then walking back down when we saw the lineup of 2.5+ hour wait. It's 1312 vertical feet (400 meters), which is equivelant to walking to the tippytop of the eiffel tower (81 story building). If you are going to do it, take a rest at each hairpin and bring lots of water. My brother and i finished it in ~30 minutes but give yourself an hour
After getting back to the ship, i watched the timing of the cable cars. It takes 2.5 minutes for one side to enter the top, exit, and get to the bottom. That means for MAX 36 pax (not always full), * 24 runs = 864 hourly capacity. when there are 3-4 cruise ships including a megaship, that could be a 4 hour wait.