I must agree with Gina. We just returned from a Discovery Cove, Seaworld, Universal, Kennedy Space Center trip. You would not do DC or SW justice by trying to do them both in one day. As Gina said, DC is a full-day park especially if you are doing the dolphin swim and you will not know what time your swim time is until you get there. You might end up with a late afternoon swim which would give you no time at SW. In addition, the new Freshwater Oasis will be open by then at DC so that will add to the reasons to stay all day. Currently, when you reserve your day at DC, they include a free admission to SW and Aquatica. You have a two week time frame to use the free admission so it doesn't have to be done back to back.
Also, the attractions/shows at SW are at specific times and you need to be there all day in order to make the most of what they have to offer and fill in between the shows with the rides.
If you have to choose between the two, IMO, I would do DC without question. But my opinion and $1 might get you a cup of coffee somewhere!