Have a look on Hellenic Seaways at the Flying Cat 4. I know they have day schedules to and from Santorini and are much quicker than the Ferry. I used them to get from Crete to Santorini during a two-tier holiday on both islands.
Only problem you may have is whether they are fully booked or running on the specific day you will be in Greece and wanting to travel!
These links should give you some further details:
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