Two companies run boat trips on the lake, but only the one located closer to the restaurant is licensed to take passengers inside the cave.
Price: 500 MKD (~€8) per person.
Cash only
~1 hour round trip
There is no need to book ahead! There are tours running every 15-20min, or as soon as enough people show up.
The section of the Vrelo Cave that you can explore on foot doesn't go very far in and isn't very impressive. What makes it worth the trip is less what you can see and more what's underneath you: the cave is believed to be among the deepest underwater caves in Europe, possibly the world, though its exact depth has never been confirmed. It's home to bats and has interesting stalactite formations, and the boat ride through the canyon to get there is a highlight in its own right.