If you want the best advice, Alex, you need to be more detailed. "late March" could mean Mean March 24 or March 31 ORRRR... Friday March 25 -- and that is Annunciation Day in Greece, a national holiday. Thus. Greeks may be seizing oppty for 3-day getaway. That could account for filled-up vessel offseason.
And why anyway do u need to get to Naxos by 10:30? U can't even check into a hotel until noon. Do u have a morning business meeting there. The Blue Star, leaving at 7:25, arrives 12:40 and is an absolute JOY to ride. You can get a "reserved economy" (otherwise known as "aircraft seat" but that has a cramped connotation and the seats are wide, far apart, cushioned, lean-back, wonderful) for about €37 ... I always ask for ones in a side compartmnt with window view. AND you can sit out on the back deck as long as you like, enjoying the view, the wake, sometimes even a dolphin? ( The fast-ferries are completely enclosed, and the salt-splashed windows have virtually no view -- also, you are trapped in theatre seating for the entire trip, no place to walk if you get restless)> On BLUE STAR, Several nice cafes on board for coffee, snacks, brunch, drinks etc. SO relaxing.
NOTE: These BLUE STAR ferries carry 1300-1800 passengers, vs. the 450 passengers of fast-ferry. Off season Blue Star usually only 1/2 full... and you could get a basic economy ticket for €4 less, and just "usurp" a vacant reserved seat. However, if it's on that hollday weekend, best to take the reserved. BTW, the "business class" on Blue Star has chairs around tables and a waiter taking y our order... but I don't like it because the seating is CLUB chairs, low backs, hard to snooze or even rest.
PS: Do NOT order tickets to be delivered in USA -- BIG extra charge, or even delivered to Hotel in ATH. What if there's a glitch?> You would have to go anyway, so you'd hav to buy again, & refunds are a hassle. If you MUST order before getting to Greece, I suggest you book your ferry ticket(s) through a travel agent who has a branch at the airport. I think they have 2; Amphitrion and Pacific Travel. Book with your credit card and ask to pick up the ticket upon arrival in airport. OR for blue star, you could pick up the ticket at the office on the pier, just beside the boat. However, for the morning BLUE STAR, I know we may sometimes be running late, and I don't want the hassle of waiting in line at a window, when people are streaming in to the vessel.
PPS: I'm always aware that the first people aboard and up the escalators to the economy deck, will have "the pick" of the best deck seats for view -- these are big BOOTHS with view of the water. Even if you have tickets, there is some slight delay on entering, while you stash your large bags on luggage racks on the car deck. If there are 4 of you.,.. delegate 2 to stash the bags, while the other 2 sprint up to the economy deck to reserve choice deck seating.