We used the Grand One yacht for an hour during the fireworks, on the day of our WDW wedding, and it was absolutely perfect. There were 10 of us, and we weren't crowded at all - in fact, we all climbed onto the front of the boat, so didn't use the interior or back at all. And, if you're going after dinner, the two restrooms inside the yacht are a nice feature.
You get a nice breeze, since it travels a little faster than most of the boats, so it was very comfortable in August. The drivers are very skilled, and got us a perfect, front row seat for both the MK fireworks and the Electric Water Parade.
If you want any food or drinks offered on the yacht, that can be arranged through Private Dining. Let me know if you have any other questions.
Cheers!
Heather W