Is it true that Swimming is the most complete cardio exercise

yea, it works out upper body, midsection, and lower body.. nearly every muscle.. and it makes you very cut and lean.

However, if you want your muscles to bulk up, the you also have to use weights.

I would disagree with that statement.  I was a swim coach and one of the things you have to remember about swimming is that you are displacing a percentage of your body weight by being in the water.  Because of that, you are not truly working your entire body.

However, for those who want to cross train, like swimming as their cardio, or cannot put too much stress on their joints because of weight or injury, swimming is a good alternative.

I swam alot in college, and even when I coached I didn't see too many skinny swimmers.  They have lean muscle mass, but being slim isn't the determining factor of success.  That's why even competitive swimmers have to cross train.