This is my KISS workout plan:

The number one priority for you is to start eating large amounts of food, every day. Eat like every day is Thanksgiving. Eat at least 5 meals a day with a couple of protein shakes and you will be getting the calories you need.

Second stop doing so much cardio. Keep the cardio to no more than three days a week, 30 minutes tops. All that swimming you were doing is burning a ton of calories that could be used to build muscle. That is why you are so skinny.

Third Concentrate your lifting on these lifts:

Bench Press
Deadlift
Squat

Why These three. Because these are compound movements that use the most amount of muscle and you can lift the most amount of weight with. More weight = more muscle. Use as much weight as you can. If you can get more than 10 reps its too light, less than four too heavy. Try to add a little weight every workout. It doesn't have to be a huge increase. Even if its just 5 lbs, that will translate to a 60lb increase on your bench in just 12 weeks.

This is how I would build the program:

Monday: Squats, Calf Raises
Tuesday: Cardio
Wed: Bench Press, Bicept Curls, Dips
Thursday: Cardio
Friday: Deadlift, Pullups or Pulldowns
Saturday:Cardio
Sunday: Off

Just do 4-5 sets of the main lifts, and maybe 2-3 of the others.
Don't wast your time on things like leg extensions or lunges or the pec deck, as they are not good exercises to build the most mass.

Your workout in the gym should last between 30-45 minutes. Were trying to stress the muscle and then give it plenty of rest. There is no need to be in the gym for any longer than this.

This may sound too simple, but I see too many people wasting their time on tons of little exercises and spending hours in the gym and having nothing to show for it. Who needs that. Spend the rest or your free time hanging out by the pool hitting on girls and getting a good tan.

If you follow this, you could easily gain 20lbs by the end of the summer, maybe more.

Give it a try and let us know how it all turns out.

Brian