A lifeguard went to the aid of two teenagers from Tumble who got into difficulties at Saundersfoot beach last Thursday lunchtime.
Ambulance control were requested to attend the beach after lifeguards advised Milford Haven Coastguard that an incident had occurred.
It later materialised that a lifeguard, Liam Murphy, had rescued a 13-year-old boy and girl, aged 17, who had ended up in the water after getting into difficulties while in an inflatable dinghy.
After the rescue, the female collapsed on the beach with suspected hypothermia and stomach pains. She was taken to Withybush Hospital, Haverfordwest.
Said a spokesman for HM Coastguard: "The coastguard services are always concerned when inflatable dinghies are used on beaches. "In this instance, the dinghy was tethered to the shore by a rope, but unfortunately the person holding it was a young boy of 13 years. "He lost the rope and the dinghy drifted offshore.
The sister of the boy in the dinghy, who was 17, swam out to rescue him, the boy stood up and fell out of the dinghy into the water. He was rescued by his sister and then both were assisted by the lifeguard. They were very fortunate that the beach was patrolled by lifeguards or it could have ended in tragedy."