I'd say typically Cancer, although I'm a Cancer and extremely extroverted so who knows? The problem with both Cancer and Pisces is they are the signs with the most variety in personalities. Cancer because the common traits that Cancers share are actually latent traits, the "shell" of a Cancer, or the part that most people see, could be anything. As for Pisces, being the last sign Pisces has picked up traits from every sign to come before it and they are very adaptable. Even with one Pisces different people will see that person completely differently because of how easily they adapt their personality to who they're with. Scorpio, being a fixed sign, is typically not supposed to be shy though, so I can't say Scorpio either.
Cancer or Pisces, although it depends on the individual. Cancer actually has the highest number of extroverts in it as compared to the other water signs, but somehow people still see the sign of Cancer as shy.