do it like this, define a skill fe skill1 as surface_type like this #define surface_type skillbalbla
than define the types water f.e. #define water 1 you could also use another type f.e. rock
than use if(you.surface_type == water) after c-trace for detecting the surface_type of the hited modell.

be sure that the entity you want to hit has a surface type (my.surface_type = water/rock/etc.)