It sounds like you're referring to the water taxi that runs between the Contemporary, Wilderness Lodge, and Ft. Wilderness Resorts. These are all located on the lake right outside the Magic Kingdom, so most likely it was the Wishes Nighttime Spectacular that they were able to watch.
You can also see these fireworks from inside the resort, from the beaches at the neighboring resorts, and from the
Pirates and Pals Fireworks Voyage. We've been on the latter, offering a private character meet and greet, food and drinks, a fun-filled boat ride, and a spectacular view of the fireworks. I highly recommend this special experience!
As for when it would happen, you can check the park schedule for the day to see when the fireworks will take place, or inquire at the front desk of your resort.
Have a great vacation!