It depends on your goals.
Assuming that you are a Competitive (may or may not be world-class) Swimmer…again, not sure if you are a Sprint Swimmer or a Long Distance (such as, Triathlon) Swimmer.
And/or, if your Health Goals are just to keep active and you feel that Running could also be taken up, along with Swimming…
Well, Swimming is a pretty Good Cardio workout.
So, is Running.
Long Slow Distance (in both Swimming and in Running) are NOT the most Effective methods of Cardiovascular Training, however.
So, if you are a Swimmer, Competitive or Otherwise…Short Distance Sprinter or Long Distance Swimmer, assuming that you are already practicing your Sport (Swimming) a lot, then, your Sports Skill therefore, increases, as will your Cardiovascular Health as well.
No particular gains can therefore, be had from the addition of Running…
Would advise you to train for Strength (Full Body, Compound Workouts, such as Squats, Deadlifts, Presses, Cleans, Chinups…).
Long Distance Swimming and Running are both, inherently Catabolic in nature and you would therefore, tend to lose muscle doing these activities.
Training for Strength (as described above) is Anabolic in nature…will help you build muscle, (will help you get stronger…will help you generate more Power, for Longer. Will also help with Stronger Kicks, and definitely at your Turn Arounds. Will lessen rates of injury and will DRASTICALLY therefore, increase your Sports Performance.
Adequate, proper Nutrition is the other Important Factor you will need to consider. Given that you are going to be participating in two Sports (Swimming and Strength Training), this is a Rather Important Facet of your Overall Training that you cannot afford to disregard.
Also, Adequate Rest and Recovery is of Paramount Importance too.
However, care should also be taken to ensure the Right Kind and the Right Amount of Weight Gain. Too much Weight Gain from a Combination of Increased Strength Training and Increased Nutrition, will likely be detrimental to performance, (Increased Drag).
A good Cost/Benefit (Weight Gain versus Performance) Analysis needs to be done, resulting in a Training Plan. Your Training/Nutrition/Rest & Recovery, should all then, adhere to this plan.
Periodic Evaluation of Actual Results should be done and Training Plan to be Adequately Updated, based on these Evaluations. This is a Continuos Process.
For more Specific Queries, I can be contacted at Srini@FocusYouStrengthAcademy.comor at +91-99414-73714 (WhatsApp))