Far North area commander Inspector Wendy Robilliard urged people to report similar incidents as soon as possible.
"Unfortunately we have a man who went snorkelling and was not reported missing until three hours after he went into the water.
"People also need to let others know where they are going and when they expect to return."
The search comes after a women got in to trouble while snorkelling at Tauranga Bay in Northland shortly after 1pm.
Police said the dehydrated woman lost consciousness, but was fine and did not require medical attention.
About the same time, police were called to another incident at Tauranga Bay where a man fell off a cliff.
Members of the public, helped by police, managed to get the man off from the water, on to a kayak and then on to a nearby boat.
The boat took the man to a waiting ambulance at the Whangaroa Marina. He was treated for a broken wrist and ankle, as well as cuts and bruises.
Ms Robilliard said this afternoon's incidents had taken up a lot of police resources and the Far North district command centre was in "full action".
"Fortunately we had some off-duty officers to help with one of the incidents."
Ms Robilliard reminded people to take care and keep an eye on each other when in or near the water.
"We are experiencing hot dry weather, which can lead to dehydration and sun stroke. These can cause people to make poor decisions or fall ill.
"Sea conditions can also change quickly and people need to know their physical limits when swimming."
