It is bad to drink pool water as the pool water can contain bacteria and parasites and other contaminants and the chlorine in the pool water can also be harmful.
Even though pool water is chlorinated especially in public pools it may still contain some parasites, viruses, bacteria etc from other swimmers even after the chlorine treatment.
People also pee in swimming pools and babies even though they have on a swim diaper may poop and the poop could leak out into the swimming pool water.
Some common pathogens which can be found in pool water include Norovirus, E. coli, Giardia and Cryptosporidium which can also cause gastrointestinal illnesses.
And ingesting large amounts of chlorine can cause digestive distress and other health issues including possible kidney failure, kidney disease, colon cancer and other cancers.
Pools can also contain other chemicals such as cyanuric acid which can also be harmful to your health in high enough concentrations.
Swallowing pool water can cause gastrointestinal illnesses and skin rashes as well as swimmer's ear and other health problems.
Swallowing a small amount of pool water accidentally is unlikely to harm you but you should avoid swallowing or drinking pool water.