I was wondering what would be the best way to practice ten ups?

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.