Great Barrier Reef snorkeling in October

The Lonely Planet guidebook I bought (... June, 1995) said that for a one-day, pre-scheduled (i.e. non-flexible for weather, etc.) trip, you want the largest boat you can easily find. That's not just for less seasickness, but because you want to know the trip won't be cancelled for low registration, and that they will have a huge "float" at the reef, with aluminum picnic tables for your meal, and a submerged bench on which to put on your flippers while seated, and then stroll to a few steps up to your shoulders!

Our boat, and I think the company, were called Quicksilver. It was an aluminum catamaran which carried over 100 passengers. And had seasickness pills in a bowl on the bar! They picked us up at our Cairns hotel.

Our trip was with Quicksilver too, and it was terrific.
They have their own floating platform out at Agincourt Reef with lots to do once there. The website has all the details. They're based in Port Douglas.
As novice snorkelers my advice would be to familiarize yourselves and generally get as comfortable with the gear as possible before setting off - it can be a little daunting to step off the platform into deep water for the first time.
If your budget can manage it I can also recommend the optional helicopter ride out to the platform or on the return. It's not cheap but it's one of those once in a lifetime experiences that we thought was well worth the cost. Plus it'll save the 1.5 hour boat ride out to the reef (or back).