Fin swimming won't give you broad shoulders because you don't use your hands.
Edit - If you do decide to get fins to play with, you can try the turtleback, which is basically a backstroke with a freestyle kick. Look ma, no hands!
Mostly it is the butterfly and breaststroke that produces broad shoulders because of the pulling action that you make in the strokes. I would stick to Freestyle and Backstroke. Use fins, and stay away from paddles. Swimming will give you long and lean muscles, not the big bulky look that weights give you from heavy lifting.