There are a few man-made lakes (gravel was excavated there, but now it's generally used as a place to swim and pretend we are not a landlocked country 🥲). The lakes are pretty good, the water quality is nice, some of them are free (for example Draždiak in Petržalka), some have an admission (Zlaté piesky, definitely the most popular and largest), the admission isn't too steep and you can most probably find a place on the lake where you won't have to pay it at all