High-speed ferries like Minoan Lines (Santorini Palace) and Seajets (Power Jet) are your fastest options, taking around 2 hours to get from Crete to Santorini.
Is a day trip from Crete to Santorini worth it?
If you are staying less than a week in Crete, then it is not worth it. There are already amazing sights on the island. If you want a white village, go to Loutro. If you have a week or more and you know you won't come back soon to Greece, then yes.