The best way to practice is to do them. You can do some push-ups on the side of the pool till you get tired, then  dive in, do your under water and do the same thing try to monitor you interval and see how you do. I wouldn't do anything less than a 1 min interval. DO NOT DO THIS ALONE. Only practice these types of events with a buddy/lifeguard who is actively watching you and prepared to get you out and perform CPR.

Give yourself a background in swimming. I only swam once in a summer league when I started training, but I spent 8 months prior to joining just being in the pool everyday and swimming up to 5000m at a time. Start off doing what you can and then work your way up. It is a matter of consistency and technique.