The deaths of 21 young Kiwi men who drowned in our waterways last year were preventable, Water Safety New Zealand says.

A third of the 78 people who died in what Water Safety New Zealand considered to be preventable drownings in 2016 were males aged between 15 and 34, despite this group making up only 14 per cent of New Zealand's population.

Water Safety New Zealand chief executive Jonty Mills said poor choices were often to blame for the tragic water deaths.

That included not wearing life jackets.

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"Young Kiwi males tend to overestimate their abilities and underestimate the risks, often not thinking about the consequences," Mills said.

It was rivers, not beaches, that were particularly deadly during 2016.

Nearly half of young men's deaths from preventable drownings last year happened in rivers.

Rivers could be "dangerous and unpredictable," particularly after weather changes, Mills said, but drownings in rivers could still be prevented.

"Rivers are not serviced by lifeguards and are often remote," he said.

"Never swim alone in rivers and always check before you jump in. Know the local conditions and know your own limits."

In an effort to reduce the number of young Kiwi males lost to drowning, Water Safety New Zealand and ACC this summer launched an awareness campaign called The Swim Reaper.

The character, which resembles the Grim Reaper, posts pictures on his Instagram account of himself at waterways.

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Water Safety New Zealand hoped seeing the character would remind young males how quickly things could go wrong in the water.

The Swim Reaper had also made an appearance on Snapchat and Facebook and Water Safety NZ used geographic and demographic data to target young males when they were at high-risk locations, such as rivers.

"We decided to hit them a bit harder in a way they understand and can relate to, hopefully to make them stop and think and look out for each other," Mills said.

"We want all Kiwis to enjoy the water this summer- whatever activity they participate in. We also want them to go home safely to their families at the end of the day. The Swim Reaper is not designed to scare families and children away from the beach.

"Our overall safety messages remain the same this summer; be prepared, watch out for yourself and others, be aware of the dangers and know your limits."

With New Zealanders starting to flock to our beaches, swimming holes and public pools as the summer starts to heat up, the family of Peter Iakapo Lemalu has called on all swimmers to stay safe.

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Lemalu, aged 17, died in March after getting into difficulty while swimming at Hunua Falls, south of Auckland.

Two members of the public tried in vain to save the teen.

Cecilia Sagote, Lemalu's cousin, urged all water-goers - especially young men - to stay safe and know their limitations.

Young males could be "too confident" and underestimate the conditions in the water, she said.

"Having too much of that, 'Oh it won't happen to me' attitude. There is also the peer pressure if you are in the water with friends. Playing 'dare' games is just asking for trouble."

It was important for them to remember "common sense prevails" and swim between the flags when at the beach, read the signs and know how to look out for rips, she said.

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Pacific Island men in particular loved the water, but some were unable to swim, Sagote said.

"Many trips to the beach are often done together with church groups so churches and other groups should be obligated to teach swim safety to kids outside of schools."