Adamant or Naughty
These natures increase attack which is what Sharpedo excels in. Although I would get rid of Surf if it is Competitive. If in-game leave it unless you can find a better to utilize Surf.
It looks as though you are edging towards a more Physical set than Special so Adamant (+atk -sp.a)
or Naughty +atk -sp.d )will do.
Jolly is also an option if you are wanting to invest in speed. But Sharpedo's HA , Speed Boost will probably be more effective.
Modest
Sharpedo can be used as a good Special Attacker since its current nature Rash increases Sp.A and decreases Sp.D.
An ideal Moveset would be :
-Dark Pulse
-Ice Beam
-Surf or Hydro Pump (It depends on if you want reliablity or power)
-Protect
