Day Trip to Santorini from Herkalion

Have you checked the Sea Jets ferry?

As mentioned Seajets has at least 2 ferries from Heraklion to Santorini. Departing fairly early morning and getting to Santorini. They continue their routes to other islands then turn around and do the same routes in reverse order so you can return to Heraklion in the early evening.

Try the ferry hoper site to see all the available timetables. Yes, seajet is the way to go, they have fast jets that make the trip in 2 hours.

However, if you cannot find updated timetables it is because it's still too early for the authorities and shipping companies to publish them and have them available for the guests.

It's a long journey for a short visit, but if you must see the Famous Caldera View, OK. One caveat, those fast-ferries are not as stable, and August-September tends to have rougher seas ... and no outside decks for the queasy folks to stand on, so it could get Messy.

AFAIK all the ferries dock way outside Fira. I stayed there for several nights in mid-October just before Covid. I reserved a private transfer and it still took an hour to get to my hotel just outside Fira. There was a horde of taxis and vans waiting to take ferry passengers up into town and it was a real traffic jam getting out of the parking area and up the 2-lane switchback. The cruise lines dock right below the heart of town. Note that you'll undoubtedly be there at the same time as 1 or more cruises so expect that everything will be very crowded, including the narrow streets.

AFAIK all the ferries dock way outside Fira. I stayed there for several nights in mid-October just before Covid. I reserved a private transfer and it still took an hour to get to my hotel just outside Fira. There was a horde of taxis and vans waiting to take ferry passengers up into town and it was a real traffic jam getting out of the parking area and up the 2-lane switchback. The cruise lines dock right below the heart of town. Note that you'll undoubtedly be there at the same time as 1 or more cruises so expect that everything will be very crowded, including the narrow streets.

There's a lot to see on Crete and IMO the views in Santorini aren't worth the hassle, especially since you won't be able to stay long enough to see the sunset (in July sunset isn't until 8.30).