I coach at a strong high school program (multiple NXN appearances in last decade). I have had several runners who swim either over the summer or winter and try to run at the same time. Here is what I have found:
1) Swimming and no running is better for RUNNING fitness than being totally sedentary (duh)
2) One swim workout and one run per day is okay at maintaining running fitness
3) One swim workout and two runs per day is too much; you'll either burn out or just not improve. The stimulus is too large for your body to adapt and you'll stagnate in both sports (though moreso running, it seems)
4) Likewise, one swim workout and two runs per day is also too much. It does not seem to be quite as bad because the runs are not interval workouts, usually.
5) As you might guess, no swimming at all and a full schedule of run training is optimal for running success. Your teammates who do this will improve more than you.
It all depends on what your priorities are. If you're 16 or older, it's time to focus on one sport at a time. I recommend choosing the one you're better at. Run in the summer and do cross country, then swim all winter. It's about an equal amount of time in the state I coach in. Or, if you are talented enough and have the desire to compete in college, pick one sport and do that exclusively from now on.
At the end of the day though, it's up to you, not your parents or your coach or random people on the internet.