It all depends on what you desire the most. no one can answer this question other than you
if you had to chose in a life or death situation swim or marching band?
personally i think you should do marching since you can swim in your spare time whenever you want. go to the y maybe. but marching band isnt a thing thats available everywhere unlike a pool. and now you have experience both marching and swim while still being able to swim whenever
but if you do prefer being in a swim team and competing against others then choose swim. are you competitive? or do you want a break and so something more in the relaxing side and still be able to swim in your spare time for fun