Focus on running. The amount of swimming you do in swab summer is minimal. Be sure you can tread water and swim (freestyle or crawl) two lengths. You will be given an opportunity to learn some basic survival techniques like how to inflate your trousers to use as a flotation device, etc.
Once swab summer is over, you will be required to take a swimming course 4/c year. So, if you're not a great swimmer, you will be taught here. If you are a great swimmer, this should be an easy A.
If you like swimming, go and work out, but don't do it to the exclusion of running. You do lots of running during swab summer, much of it on pavement, so shin splints and foot blisters become problems for lots of swabs. Swimming will not help you avoid shin splints and blisters. Running should. Just don't overdo it. You don't want to begin R-day already suffering from shin splints. Start slow and build.