Be ready for that. Floating tips: If you are having a difficult time floating or treading, try to breathe differently when you tread. Keep air fully in your lungs -- hold for 4-5 seconds.

Elements of a drownproofing test (tread, float, bottom bounce, etc.) are great rest exercises between fast swimming sets of 100-200 meters in length. See this sample workout: Repeat 10 times.