I attended a Grand 1 fireworks/dessert cruise last summer as a guest. It was a lot of fun. I don't have many photos shared of that cruise, but I do have some: click here.
All of Disney's watercraft have size limitations. For our Grand 1 cruise last summer, the guest list was capped at 10 people because we were having food & beverages served aboard. Sometimes more folks are allowed on the boat, but it is entirely up to Disney and you have to abide by their rules.
I have also been on a couple of fireworks cruises in other areas of WDW. My family has watched both the Disney-MGM Studios' Fantasmic fireworks and Epcot's Illuminations fireworks from watercraft. When you book those boats, Disney will let you know what their capacity is.
Click here for the official WDW "specialty cruises" webpage. I think a specialty cruise is a wonderful idea. My family has always enjoyed them.
My sister and I did the specialty cruise for my parents on their anniversary. and it was so beautiful they enjoyed it very much. It was about 8 of us..