If you don't have a cabana there's no situation that tipping would be appropriate in, all of the food you get yourself, and you don't pay at the snack bars so your chances of having any money on you to tip are unlikely.
My husband and I did rent a cabana while we were there on the 18th and I had asked the same question on Facebook. I got a reply from someone who works at DC and she said that they are techinally not allowed to take tips so we didn't leave any. However I have to say our cabana attendant was wonderful and I wish we could of.