Congrats on the flight slot. Back in 1990 I worked in the pool while waiting to start API. The best advice I can give you is to keep going to the pool, mostly so you're used to being in the water. As far as swimming under water is concerned, as long as you're not a wuss, you'll hold your breath as long as it takes to stay in the program. While you in the pool practice treading water a lot. Try it using hands and legs, and try it using only your legs. The idea is not to make you a SEAL, but to build up leg strength for when you have to tread in full flight gear. The mile swim is not big deal because they give you a ton of time to complete it. I used the breast stroke and finished in less than half the time they give you. Don't worry too much about the water survival stuff. They don't just throw you in the water with full flight gear the first time. You'll build up on just about everything you do. First in swim suit, next with flight suit, then boots and SV-2, lastly putting the helmet on. The instructors there are very good and if need be you'll be held back and all you'll have to do is concentrate on your swimming until you get it down. And one last thing, the swim stuff is pass/fail so no part of it factors into how you rank in API.
Hope this helps. If you have any more questions don't be afraid to ask.