A 72-year-old woman was among several people rescued after trying to cross to a small uninhabited island.
A rescue helicopter, lifeboat and coastguard rescue team were called to Sully Island, Vale of Glamorgan.
The woman was taken to hospital after injuring her ankle wading through waist high water when cut off by the tide on Saturday afternoon.
Swansea coastguard urged people to heed local warning signs about the dangers of crossing to the island.
The woman was taken to hospital in an ambulance.
Bernie Kemble, duty watch manager, said: "We strongly advise that members of the public take account of the many signs and warning notices that are placed around the causeway advising on tide times and when to cross.
"When the coastguard team arrived they discovered that the water was waist deep, and although other members of the public managed to get out this particular woman fell and hurt herself."
Sully Island lies midway between the towns of Penarth and Barry.
Access to the island, which is about 450m (490 yards) off the mainland, is on foot across a causeway at low tide from the car park of the Captain's Wife pub.