It's a race. Sail fish, Mako shark and Generic Tuna (not to scale). Achieving record speeds of 68 mph, 60 mph and and 43.4 mph respectively, according to my google search. Next time you're driving along at 68 mph, picture a sail fish keeping up in the lane next to you.

Notice how they're all counter-colored. That is, darkish on top, lighter on the bottom. It makes sense when you're living in the open ocean and trying to be camouflaged. From the top looking down in the water, it's dark to the stops of fish are dark, and looking up in the water, it's light so fish bellies are light. Same coloring system goes for penguins. Actually the point of this image was not to show examples of counter-coloring, but to illustrate the fact that sharks are fast swimmers. Their speediness is partially due to their skin structure.