Well, I may be the only person to feel this way, but we did not think that the Illuminations Cruise was worth it. We were disappointed with the view and felt that we were too far isolated from the show. Our usual way to view Illuminations is to stake out a spot 1-2 hours ahead of time where we're right next to the rail, and often have fireworks being shot off from directly in front of us. The boats have to stay under the bridge which is just too far back to me. Also, we were sharing with another DIS family, and we were seated at the back of the boat. That might have had just a bit to do with it, but I don't think the view from the front of the boat would have been THAT much better.
I can think of 2 reasons why I'd recommend an Illuminations Cruise.
#1 If you're travelling with a large group (10-12) people, this would give you a semi-private way to view the show and make sure you have adequate space for all of your group.
#2 If you're viewing Illuminations on a night that you're not going in to a park and using up a day on your pass, this can be a pretty good deal to be able to view the show for around $10 - $15 per person (may be more with the price increase), assuming you're sharing with others and the boat is full.
I have not been on a Wishes Cruise, but from what I can tell, I would not recommend this because you're not guaranteed to be able to hear the music, which is such an important part of this show. Also, it seems to me that you'd just really be too far away here, too, but I can't speak from first-hand experience.