How did dolphins acquire their dorsal fin?
Short answer
I reckon that the dorsal fin of sea-faring mammals are a new development altogether.

Background
The presence of a dorsal fin is one of the defining characteristics of fishes like sharks. Mammals that returned to the sea generally lack a dorsal fin.

However, killer whales and dolphins, while being mammals, indeed do have dorsal fins. In contrast to the dorsal fin in fish, these structures do not have bones (Fig. 1). Instead, the dorsal fins of mammals are made up of dense tissue, somewhat like a ridge of thick, folded skin. Hence, it is not a 'revived structure' that was 'activated' from a latent state in mammals.

Hence, given that mammals returned to sea later in evolution and lacked a dorsal fin on land, I would deduce from these data that dorsal fins are an example of convergent evolution where analogous structures with a similar function (like in fishes, the dorsal fin acts as a stabilizer or a rudder in mammals (source: The National Marine Life Center) develop from unrelated bodily structures. Hence the genes involved may, and likely are, different.


Fig. 1. Dorsal fins of a killer whale (left) and in a fish (a redhorse on the right). sources: Sea World and Mosxostoma