I also subscribe to the "David is an Epic" theory. But I think his gift(s) are going to be more in-line with, as others have mentioned, inpsiring people (or manipulating, potentially). He wouldn't notice it immeditaley because it's not something visible or manifests anything, and several people in the book had already commented on his persuasiveness.
According to Regalia, Calamity did say that David's gift would be "thematically appropriate". I always thought it was odd that Regalia would assume that meant water powers, as Epics don't appear to be gifted powers in line with their fears/weakness. Not only that, but water (or large bodies of it, if you prefer) was a relatively new thing for David, and hadn't played a major role in his life up until this point.