Well Martin... for starters is Rui not Riu  :-) 
Now let me see if i'll can explain myself, too:
- drybrush with the mix you used from the top of the sail downwards;This should be done with the same colour as you painted the sub inicialy. Under the sail and most of the sub, leaving the areas near the waterline...
What do you mean by downward strokes?From the point where you put the brush, down  

This way it will look as the colour is fadding away... 
This is something like painting chips on armor models?No. Please see the picture that Mike Taylor posted. As you can see right on the bow it's not like chipping paint
So the rust is only near the waterline? I thought these oscars had a special layer which avoids rust.Here it's not a question of rust, because, as you mention it so well, modern subs have special layers and paint to avoid rust, but .... (and there's always a BUT) pipes and plumming inside don't have this treatment, so if you have a point where water pours out from the sub, there's a heavy possibility for rust to be poured out to, leaving rust on the way down - not on the surface but from the residuos inside... 
Where do I need to pay attention to with the scopes? Just paint them  :-)
Hope that this time I was abble to explain myself   
 
 PS: 
To GarryBeebe:
Garry, when the Dreadnought Campaign starts I expect "payback time" :-) , since I don't build a battleship since I was 12 or 13, and at that time I built the Heller Cadet 1/2400 (??) Bismark and Tirpitz - which, by the way where left out in the Campaign "scale window" 
Skipper