Water is potable and treated in Cuba but, in general, tropical water has a different pH and microbial content than “northern” water. You know your own system and what you can take. Bottled water is easily found for sale in many stores. If you’re drinking tap water and have a reaction, it’s usually a self-limiting diarrhea which can be treated by switching to bottled water for a few days.
For your information, it’s important to know that the Ministry of Public Health has alerted all Cubans that it’s safest to boil their drinking water, if they’re drinking from the taps. Because we’ve had so many tropical storms and hurricanes during the past several years, it’s affected the usual high quality of the water.