Ferry seats - I'm struggling :-)

The morning ships departing from Piraeus for Naxos include the Blue Star Delos ferry that carries up to 2,500 passengers and the high-speed Seajet Eurochampion Jet 2 which carries 1,500 passengers. A catamaran of that size is not going to have issues with stability on the water.

Much smaller boats sometimes do- but catamarans of such large capacity are as stable as the large “ferries.”

Since October is the month when the tourism season begins winding down in the Greek Islands, I don’t think you’re going to have an issue with finding seating together on October 1.

Another site to book ferry tickets is www.DirectFerries.com

If you wind up taking the conventional ferry rather than the high-speed one, I would probably pay for the lounge seating. It's going to be a nicer environment. Even if it's not that crowded, you'll be comfortable in the lounge, and it's not that much more expensive than the regular seats. After all, the ferry ride is going to be at least 5 to 6 hours or thereabouts, so you might as well be comfortable during that long trip.

If you want to spend most of your time in a quiet lounge on the Blue Star to Naxos, book the lounge seats. There are no specific assigned seats, you can sit anywhere in the lounge and won't have any trouble finding seats together. They are very lightly occupied.

Personally, I always book economy. I always find a way to sit somewhere in a quiet area if needed and I like walking around the ferry and the economy decks. So even though the extra money for the lounges isn't expensive, I don't see the point given how I’d actually use it.

In any case, even if you do book a seat in the lounge, you can still access the economy class areas and sit there.

I would recommend taking only the economy ticket. The ship should be Blue Star Delos, which has many outside decks and 1st of October will most likely still be warm enough to use them. If you'd have enough wind after some time, you can still go inside and find some place there.
Outside decks have the advantage, that you can arrange the seating more or less the way you like and really sit together on a table.

I agree with all above on booking economy seats. The "reserved economy" (lounge) areas are huge, and in October will be very lightly occupied ... if you or your inlaws, wish to come in from the back deck after a few hours, to sit in a leanback upholstered seat with headrest & footrest, just take a walk around & find what suits ... many vacant seats. I favor the smaller side compartments by or near windows, rather than the center, which is like a big movie theater, lighted. with TV going.

It's wonderfull to sit on back decks & view the sea, you might even spot a dolphin in Autumn! There are 2 back decks open for enconomy; here's a strategy for getting best seat (it's first come, first served). Get to the pier early, so you'll be among the first in line; when they open the gate, everyone hustles up the ramp, so if your inlaws take longer, you 2 hurry on ahead to the main (vehicle) deck. There, you will leave your Biggie bags (take with you your smaller backpack/shoulder bags w camera, valuables etc). Luggage is perfectly safe on vehicle deck; it's locked while underway. There is shelving for Naxos bags OR you can pile your 4 against a wall for quickest access. Your husband can handle this, while you scamper up the escalator, then up steps, to access either back deck -- if nimble, you have best choice of tables/chairs, or if luckiest, built-in booth seating. Take a Big Scarf to wave, to attract the other three as they arrive.

After the Paros Island stop, NAXOS is about 1 hour ahead. It would be nice to stand on deck while the boat turns around, to gaze at the wonderful Causeway arch as you dock. However that's chancey, because you'd have to descend 3 flights, collect luggage, and disembark very quickly -- because these ferries have a SHORT dock-time! Slower people could fail to get off & have to go on to Santorini! Thus, when the PA system announces Naxos, get moving. An elevator accommodates a few, if inlaws are less mobile... stairs for the rest. On Vehicle level, in the gloom you'll get your bags and stand next to a huge 18-wheeler with the crowd, staring at the ramp-door. Finally, the thump of ferry hitting dock, ramp slams down and -- Voila! the beauty of Naxos TOwn and the hills beyond will greet you. Bon Voyage!

Really travelling in Oct there is no need to pre book tickets and if you do, book direct through the ferry company not a third party.
It’s very unusual for someone to miss disembarking as they do not allow others to board until the ramps are clear of passengers getting off.