I would certainly say it's worth a call to see if there is availability, it's also worth calling a few weeks before you travel as they may have had late cancellations, you can check up with the CM on the phone to see what the cancellation policy date is, many people would leave it to the last few days to cancel but still get their money back.
Few things for those who've not been before
1) Do get there early. It should give you the pick of swim times, means your processed in a relaxed fashion before the rest of the guests and allow you to get a nice spot on the beach near the dolphins. It's not that space is at a premium, but if possible you'll want to try to get a perfect spot with some shade, room for chairs/loungers, close enough to see and hear the dolphins all day and easy access to the other attractions at the park
2) make sure you check out all the activities at DC, the bird feeding is cool and sometimes they have small deer that will come and feed from you. Check out what time they feed the stingrays, my DD really enjoyed in helping with this.
3) Make the most of your week at SW, I find it really great to be able to take your time to explore the park spread over three or four days at a relaxed pace. Damn I nearly got my money back in free beer
