I've done the MK cruise twice, leaving once from GF and once from the Poly (our choice). You cruise around the lake, and depending on your captain's knowledge can learn a lot of info about WDW history/facts. We "parked" across from castle in time for the fireworks and had a terrific view and enjoyed hearing the music. It was truly magical, like the show was done just for us.
While I've never done the Illuminations cruise, the MK fireworks one is more appealing to me. I've been on the International bridge where the Illuminations boat parks for the show, and the people on the bridge are looking directly down into the boat. A couple people on the bridge joked that they'd better be careful with their pop sitting on the ledge, because otherwise it would land on someone in the boat.