// Create Event
SWAY_HEIGHT = 64;
MAX_SWAY = 2;
swayspeed = 0.02;
sway = 0;
spriteid = sprite_index //or whatever you want the sprite to be

//Draw Event
sway+=swayspeed;

for (p=0; p<SWAY_HEIGHT; p+=1)
{
  draw_sprite_part(spriteid, 0, 0, p, sprite_get_width(spriteid), 1, x+MAX_SWAY*sin(p/SWAY_HEIGHT*2*pi+sway), y+p);
}
draw_sprite_part(spriteid, 0, 0, SWAY_HEIGHT, sprite_get_width(spriteid), sprite_get_height(spriteid)-SWAY_HEIGHT, x, y+SWAY_HEIGHT);