Is it true that no masturbation before a swimming competition gives you more power, strength, and alertness?

Not really: No, there's no facts to back this up. Competitive athletes have always been very superstitious folk, and boxing coaches even today coach their fighters that masturbation (or sex) will make them weak. Depends what you want to believe.

Just not true: Superstition is common. Coaches also urge athletes not to have sex because relationships make life much more complicated, take time away from training (and college studies), and can play emotional havoc if they fail (and they usually do), making the athlete less effective. During times in which life needs to be focused, imagine a future wonderful relationship and enjoy being who you are.