we also very much enjoyed it there.
I think we went the very first year they opened (whenever that was).
We scheduled our dolphin swim for very early in the morning and arrived at the park pretty much at opening.
We had to pay in advance (when i made the reservations), but on the day we arrived, my daughter couldn't do the swim, and they refunded the dolphin swim charge immediately without question and were very sweet about it.
On our swim, one of the women in the group didn't know how to swim and they were able to work around that. So not swimming isn't an issue for the dolphin swim part.
Also, as i recall, they gave everyone a scuba-diving swimvest to wear, which helps with buoyancy.
We also didn't stay the whole day. We stayed through lunch and then left (we never hang around any waterpark for long - we live on the mediteranean, so we tend to try to avoid the sun rather than seek it out).
in addition to the dolphin swim, we very much enjoyed their version of a lazy river as well as the shallow bay area where the stingrays are (when we were there you could feed them, i don't know if you still can or not).
It's really a beatiful place, and the fact that they limit the number of guests makes it all that more wonderful.
Our ticket included entrance to Sea World (i think for several days). So we went to Sea World, which we didn't like at all, so we didn't make use of the multiple day aspect of that ticket.
But Discovery Cove all on its own is definitely worth the price of entrance. And the dolphin swim was very nice (and in advance of the swim there was a very interesting prepartory lecture about them).
One side note that was interesting to us. When we were on the dolphin swim, one of the workers suddenly came up to us to ask us where we'd gone to university - it turns out we all went together. He's one of the scientists at DC (he has a PhD in marine biology). How he recognized us after 25 years is beyond me. In any case, he explained that there are many high level scientists working there, so it's not just another waterpark. They actually are involved in research as well.