In general, cacti need water to grow. But where I used to water in May, during nice sunny or even warm wheater, it is now a different situation. Some Opuntia (mostly polyacantha, fragilis and its hybrids) need early water to flower. So the Opuntia's get water early March, and fragilis and it hybrids late February. Even than most fragilis are reluctant to flower. This is no different from outside.
If you don't repot every other year, the quality of the water becomes more important. Rainwater is the best. But if it stands still in a basin, aerating it with air is necesssary to oxygenate the water. That way the water becomes saturated with oxygen, which is much better for your plants. If you have to resort to tapwater, acidifying gives you at least a moderate pH (somewhere between 6 and 6.5). You can use vinegar, or sulphuric or phosphoric acid if you use bigger quantities. How much you have to use (in ml per liter) depends on your water. Before I used about 5 ml of a 10% solution of suplhuric acid on 10 liters of water, which gave me a pH of around 6.2.
Don't use hydrochloric acid! The chloride will kill your plants.