A sea current was sweeping over a sandbar and meeting the outgoing river current, the man said.
It swept the young fisherman "in to deep water and trouble pretty quickly".
"When I realised I couldn't physically help him -- and I felt terrible about that -- I got my phone out my pocket and dialled 111," he said.
Police rushed to the scene and helped the man ashore.
He managed to stay calm and remove his waders and swim with the current to safety.
A rescue helicopter was stood down after the man got safely ashore.
Officers commended the actions of both fishermen.
"The other man did the right thing by alerting emergency services, giving a running commentary of the situation/location and not attempting a rescue himself until the man was in shallow water."
The witness said the man had a lucky escape.
"He was exhausted. I think he did very well, he didn't panic, he bided his time, he could see me on the beach with phone to ear, so he knew steps were being taken to help."
He didn't think he was doing anything particularly wrong, and feared it could happen again.
"It could happen to anybody, and more than likely, will again, what with the way the conditions are there at the mouth."
The more experienced fisherman urged people to be careful whenever around river mouths where currents and weather conditions can affect the underwater terrain and footing can be lost unexpectedly.
Police echoed his warnings.
They recommend all fishermen, particularly those fishing alone, wear life jackets.
