What are the chances that a tampon will release the blood flow in the water?

Chances of leaking in the water while wearing a tampon, also using them correctly such as by inserting a fresh tampon before swimming and using correct absorbency, is fairly low but still possible. If worried you may want to consider using a menstrual cup, these are internal like tampons but hold far more than tampons and have light suction so don't leak like tampons.