Giangilberto from Milan here — Maggiore is our weekend lake, so this one's home turf. Short answer: no advance tickets needed for the boats. The "cruises" everyone actually takes are the regular public ferries (Navigazione Lago Maggiore) that shuttle between Stresa and the Borromean Islands every 30 minutes or so. You buy at the ticket window on the Stresa waterfront that morning; get the day ticket for the islands zone and you can hop on and off all day. Early September is perfect — past the August crowds, boats never "sell out."
What you CAN book ahead — and I would — are the entrances on the islands themselves: the palace and gardens on Isola Bella and the botanical garden on Isola Madre (both Borromeo properties, there's a combined ticket online). Queues in September are gentle, but with a group of six it's nice to walk past the window.
My favorite version of the day, done many times: morning ferry to Isola Bella when the palace opens (the terraced gardens are the postcard — white peacocks included), then hop to Isola dei Pescatori for lunch — it's a tiny fishing village and the whole point is grilled or fried lake fish at a table over the water; go by 12:30 or book, the good terraces fill fast. Afternoon on Isola Madre, the quiet one — an English-style garden where you'll wonder where everyone went. Last ferry back to Stresa with the light going gold on the gulf. That's the "cruise," and it beats any packaged tour sold online.
One more option that works specifically because you're six: the private taxi-boats lined up on the Stresa waterfront. Per boat, not per person — split six ways it's surprisingly close to ferry prices, they'll do a custom island loop and wait for you at each dock. Just agree the price and the plan BEFORE you step aboard; that's the local way.
Are you thinking full palace-and-gardens day, or more of a lazy on-the-water afternoon? The right plan changes a bit.