The restriction applies to all activities on or in the water, including swimming, paddleboarding, kayaking, and any other water-based activities.
"We ask that visitors respect the area and do not remove, disturb or interfere with any objects or features revealed by the low water levels," it said.
The beauty spot car park remains open to visitors but "for your own safety", the national park said, "you must not enter the water under any circumstances".
The whole of Wales was placed into drought status last month in response to record-low river levels, dry soils, widespread wildfires and mounting impacts on wildlife, farming and forestry.
