I swam competitively all through high school, and a bit before. My workouts were pretty hard, im not gonna lie, and i practiced around 7 times a week. You probably wont see an increase in muscle as much. If anything your pecs might get stronger and abs, but thats about it. Probably also an increase in cardio, lose weight, and as you said get a flat stomach.
I swim competitively. It doesn't build muscle as such, but what it does do is give you all over toning, so your muscle can actually be seen! I train at least 3 times a week but it means I can get away with eatting utter crap! Also you end up with a really nice back, broad shoulders and a nice chest.
I'd say swimming is the best form of all round exercise. Stick at it and good luck!