Surf -
Ok, surf is helpful outside of the battle and also the strongest HM of the game,
It I would say has Exceptional Power and can be useful during parts of the game
So Surf wouldn't be a bad choice if it is what you chose.
Hydro Pump -
Hydro Pump is awesome and much stronger than Surf it would be an awesome
Choice during a Gym/Pkmn Trainer Battle but if you want a good ouside and
In battle move It shouldn't be Hydro Pump. Hydro Pump Though is a great battle
Move.
As You can See Either would be a great choice :D
For Battle: Hydro Pump
Outside & In- Battle: Surf
**Hope I helped :)**