A friend of the two men said the they were from northern India where only about 20 to 30 per cent of people knew how to swim.
"I think it's very important to learn to swim here. If you're staying in New Zealand, learn to swim. Because accidents, if something happens, there's no way to come out of the water," the friend said. Mr Tretheway has pushed the importance of learning to swim, not just for a person's own safety but for others.
"We're surrounded by water, it's a life skill you have to have to feel safe," Mr Tretheway said.
"It's just that feeling that you can look after your own children."
Both Baywave Aquatic and Leisure Centre and Greerton Aquatic and Leisure Centre hold beginner classes specifically for adult swimmers.
"Out of my 20 years' experience I get the most enjoyment teaching adults," Mr Tretheway said.
"A lot of adults never learned at school or had the opportunity to learn as a child, it's actually quite sad."
Mr Tretheway said the stigma around adults learning to swim was unfounded and unnecessary.
"A lot of people might be embarrassed but as soon as they can overcome that and step forward, they're halfway there to learning," Mr Tretheway said.
His most memorable moment was a class consisting of an 83-year-old man who decided he finally wanted to learn, and also a 17-year-old "who realised she was missing out".
"It just made it great. We judge no one, learning to swim is one of the greatest life choices we can make," Mr Tretheway said.
Mr Tretheway said the classes were restricted to about six people, and at present there are 31 adults attending the swim sessions.
He said there were "very few Kiwis", but the classes consisted of people from overseas who knew they needed to learn.
"It's brilliant, they recognise it's something you need to know in New Zealand," he said.
Bay of Plenty English Language School (BOPELS) manager Liz Harper said that the majority of their students come to Tauranga with little or no swimming skills, but they have acknowledged this and seek to educate their students on water safety.
"As part of this we have taken them to surf lifesaving for further instruction," Ms Harper said.
Statistics from Water Safety NZ showed 83 people have drowned in New Zealand this year, compared to 78 in 2014.
They also showed the Bay of Plenty as the fourth worst region for drowning fatalities between 2009 and 2014.
Water Safety New Zealand CEO Matt Claridge said the recent drownings in Tauranga "are a tragic reminder of the importance of learning both essential water survival skills and to swim".
"New Zealanders love to spend time in, on and around the water so it's absolutely essential that everyone, whether a young child or an adult, learn the necessary skills," Mr Claridge said.