Can you get a venereal disease in a swimming pool in cancun if people have had intercourse in it?

No. No, the chlorination kills viruses as well as bacteria. Yes, but only if you where part of the mix. Don't worry, pools do not transmit stds. If so, we would all have it!
No. You can not catch an std by swimming in the pool. Not sure i want to swim in that pool though. Sexual contact is required to pass an std from one person to another.