Go swimming -- this is the quickest and easiest way to get a lifeguard's body fast. Swim laps in a pool for one hour each day for six days per week. Break it up into two half-hour segments if you cannot do one hour at once during the first few weeks.
Do other water exercises, such as moving a beach ball in a circular motion above your head, or trying to push it under water, to build your muscles (this should be done at one to two minute intervals for three sets per day and three days per week). Do regular exercises in the pool too, such as walking -- the force of the water pushing against you will help you get lean. Walk for 30 minutes to one hour per day, six days a week.
Go running for 45 minutes each day, six days a week. Lifeguards must often run to keep in shape, as well as help swimmers during an emergency. This gives them muscular thighs and calves, and increases their cardiovascular endurance. Run on sand, if possible, as this makes your body work harder to maintain balance, and builds muscle faster.
Take yoga classes for at least 45 minutes a day, three days per week. Yoga helps build the same long, lean muscles that lifeguards often have, as well as giving you better flexibility.
Increase your protein intake to build muscle. Eat foods such as lean chicken (no skin) or turkey, beans, nuts, low-fat milk, yogurt, and soy products.