I was going to use the website, www.aferry.com but keep getting the message that the ferry is only available during the summertime. I put in the dates, June 25? Comments?

The ships are run by Jadrolinija (www.jadrolinija.hr) and Blue Line (www.blueline-ferries.com), both running all year. Jadrolinija tickets from Ancona can be booked through their agent, Captain Amatori (www.amatori.com). There is also a fast ferry service in the peak season, run by SNAV (www.snav.it).

Thanks so much. I will google.

Geoff,

How necessary is it, in your opinion, to book a ferry going from Ancona to Split ahead of time? Do they ever take walk-ups (this would be in mid-June, though, so "high season," I suppose). The reason I'm wary of doing so is because I don't know exactly what time I'll arrive in Ancona.

If you miss one ferry you've booked, is there a possibility of using that booking to get on the company's next ferry?

Blue Line charges peak season fares from 19 June, Jadrolinija from 26 June.
I doubt whether the ships are ever completely full, but there may be a shortage of cabins. In my experience, even if the cabins are all booked, something will be available on board (people book then miss the ship or change their plans).
Blue Line's ANCONA is the biggest ship, with 492 cabin berths and 172 aircraft-type seats. Jadrolinija's DUBROVNIK has 459 cabin berths and 384 aircraft seats.