We did the MK fireworks cruise in 2012. We ate dinner at Chef Mickey's (nine people, three generations) and picked up our cruise at the Contemporary marina. Our captain was awesome - every time we have done this, we have had a wonderful captain. I can't remember the order we did this in, but we cruised around Bay Lake and got to see the Electrical Water Pageant close up. We also went to Seven Seas Lagoon, where we stopped to watch the fireworks. We did pay the extra money to have the fireworks music played on the boat, and that was well worth it. We had a wonderful view of the fireworks, though we couldn't really see the castle projections, which was fine. It was also a huge bonus to arrive back at the Contemporary, get in our car, and go back to our resort without dealing with the crush of people leaving MK.
The Illuminations cruise we have done more, last time was July 2014. We pick up the boat at Yacht & Beach Club, then they usually take you on a tour of the canal by the Swan and Dolphin, down to DS, then turn around and come back, down the canal to EPCOT, near the International Gateway. The Illuminations fireworks cruises park right there, either right in front of or almost under the bridge between UK and France. It's a great view of the show, and you can see everything. Depending on how the wind is blowing, you may get a bit of smoke blown your way, but it's nothing terrible. In 2014, our captain took us to Illuminations first, then took us over to DS to watch the Frozen fireworks from the water. We saw those fireworks at the Frozen Dessert Party inside DS a few days later, and I have to say, the view from the water was better. However, you can't see anything that happens down closer to the ground, just as with MK, which may or may not be an issue, depending on personal preference.
I would imagine that this year, the Illuminations cruises would go there at 9:00 and then head over to watch the Star Wars fireworks at DS at 9:30. There is no extra charge for music on these cruises, as you can hear the sound of the show, no problem.
The other fun part about the Illuminations cruises is that when you are traveling through the canals, there tend to be a lot of people walking along them who will wave to you. If you go with a celebration package that includes a banner and balloons, even more people will wave and call out - we did this one year with my mom for her 65th birthday and she thought it was the cat's meow that so many people waved and wished her a happy birthday.
The fireworks cruises are a special experience. We usually go with extended family, so we come close to the maximum of 10 or 12 people on the boat, but if you are in a smaller group, I know people post on various Disney boards about interest in sharing the boat and costs.