The pair were airlifted to safety about 5.30pm by a rescue helicopter.
A boat, owned by a Glenavy fisherman, was also placed on standby to assist.
The man and boy were not injured.
An Oamaru woman at the scene, who asked not be named, had seen similar incidents in the past and said people needed to take more care when fishing or boating in the area.
"I actually think they should be very mindful of the tides.
"There are quite a lot who go out there and don't know what the tides do. They don't know where it's safe.
"They go out there at low tide and don't realise the water is coming in and get stranded.
"I've seen a lot with no life jackets, not bringing appropriate oars, losing oars and very limited boat skills. Since Christmas ... I am aware of half-a-dozen instances where skilled boaties have helped people out.''