I've taken Illuminations cruises as part of a group twice. They were a lot of fun. But that being said, I'm not so sure I think it's worth the price now, even when split by 10 people.
When I did it, I think it was $15-$20 per person and that included a tip for the boat captain. You could arrange with Yacht Club room service to get food catered for the trip, but they allowed you to take on your own food and beverages, including alcohol. So we'd bring on a little cooler with soda and beer in it, and bring along our own snacks.
Since then, Disney has jacked up the price, gives you a few snacks and sodas, and forbids you from bringing your own food.
I think they only time I'd do a fireworks cruise again might be if I win the lottery or something and I take a group of friends and family out on the Grand I for fireworks at the MK.
-Rob