Just my 2 cents:
We did the dolphin interaction/swim when our family went in June of 2014 and the kids (at that time the were 6, 10, and 13) loved it. It was as described above and really enjoyable. And yes we rented the cabana package that included all the photos and a DVD. It was pricey to say the least. We are going again this summer and I was braced for a fight to do the dolphin swim again and was pleasantly surprised when they (all three of them) said "We are so excited to go back. As long as we get to swim at the reef with the rays and fish and visit the aviary, we really don't need the dolphins. It was nice, but the rest of the day in the reef was the best part!" From someone who has been there and done it, I would also vote that $800+ CDN is too much money for the dolphin time. My kids would agree. Discovery Cove is so awesome, that I would really expect you to have a wonderful day there with just the day pass. Additionally, the dolphin swim took up a chunk out of the middle of our day with the pre-dolphin safety film/briefing, the actual time in the water and then looking at and choosing photos. Without the dolphin swim interruption, you will have so much more time to relax and enjoy all of the attractions at DC. Have a great trip!