what are somethings bad that can happen when swimming?
The benefits of swimming typically far outweigh anything bad that "might" happen. Bad things should not happen if the swimming pool is properly maintained. For that reason, when swimming you should test the free chlorine and pH of the water, wether it is your pool or not, to be sure that you do not become ill with a recreational water illness (RWI). Also, be sure to observe all safety notifications (diving rules, lifeguard rules, etc.).