We just got back from our first time at DC and our 4yo ds LOVED it! He couldn't do the dolphin swim, but they provided a babysitter so the rest of the family could all go together. They also allowed him to come into the water after our swim to meet the dolpin and take a family photo.
He really enjoyed the coral reef and ray pool too, and picked up snorkeling really quickly.
This was the absolute highlight of our trip, I say go for it!
Tracy