If you're preparing correctly now, boot will get you out of shape. You need to be running, swimming, and working out (with a focus on long term muscle endurance) every day. 2-4 hours a day. Try to work your way up to 20-30 miles of running per week, 3000-5000 meters of swimming (*more if you want... but swimming laps gets old*) and thousands of cals each week. Also, you need to be confident in the water... confident while your body is telling you to get oxygen. Get yourself accustomed to hypoxic (look the subject up on the net) activities in the water. Note: do not train for this alone, and do not train for it with some highschool lifeguard on duty. Have someone personally watching you at all times who knows the dangers of hypoxic training and can get you off the bottom of the pool. If you don't have access to that kind of supervision... find a way to do it out of the water.
Note for cals: try doing different sets of push-ups, pull-ups, dips, arm-haulers, abs, and legs with as little rest as possible between them.
hope this helps
FOW