I was a swimmer back through high school and I was doing a swimmer (female) through most of college :woohoo:. From that wealth of knowledge came one thing. Eat boss. I was consuming 3000-3500 calories a day and not gaining an ounce of weight. You need to be hitting 4000 calories a day at a minimum. I know in high school we swam 2-2.5 hours a day after school, and in college they had practice 3-6 hours a day (they had two a day 3 hour practices fairly often). Those guys had to down calories like it was nobody's business just to stay as muscular as they were.
Since it's your off season, I would try to hit 4500cal/day. Learn to love peanut butter. I would also research a 5x5 routine. That's going to give you the most compounds exercises along with heavy weight. And there is nothing wrong with increasing legs, back, and chest in swimming. Plenty of the guys on the swim team were sitting at 190-200 lbs, but they were tall and incredibly lean. So research the 5x5 routine, you'll find a ton of info on the web about it. And start trying to find out how to include more calories into your diet. A quick way is to add 2 tablespoons of olive oil at the end of every meal. An easy weight gainer is just whey protein and ground oatmeal (use a coffee grinder). If you have extra time you can add more to the shake. And also eat peanut butter for snacks...spoonfuls of peanut butter.