You should replace Water Gun with Surf.
The reason for this is pretty simple. Surf has 55 greater BP than Water Gun.
Why I don't suggest replacing Rage with Surf is because, although it has low power, having two moves of the same types doesn't really accomplish anything in-game. Later on, you can replace Rage with a different move.
It is also necessary to have atleast one Surfer because you'll need it to progress further in the game.
Hope I helped!