Gaining

Keep moving!
Be careful not to hurt yourself and take Glucosomine to keep your joints healthy.
Do not overdo it but be sure to do enough.

I still run nearly everyday and do some exercises, in addition to ice skating once a week. I have put ten pounds this summer, up to 230 lbs. So yes you can gain and still be active, as several others have pointed out already, just don't over do it. Good Luck

Basically eat what you're eating, but more. Make sure that you're still consuming enough to have a surplus at the end of the day. I'd pay close attention to protein that you're taking in. Protein is what you need to repair your body (muscles after swimming for example), but when there is nothing for the protein to repair it drops a nitrogen and stores as fat.

As far as your carbohydrates go, things like white bread and lighter colored pastas are much more efficiently turned to energy in your body, so they're perfect for fueling quick workouts, but also get stored as fat much more quickly than wheat breads and such.