A smart phone in a ziplock bag could be the difference between life or death out on the water this summer.

Maritime New Zealand is reminding boaties to ensure they've got at least two forms of waterproof communication devices.

In a recent case in Nelson, a father and his 10-year-old son were left treading water when their boat was swamped by waves.

National Advisor for Maritime New Zealand Baz Kirk said the father called emergency services on a smart phone his wife bought him, just the day before.

"It could've gone very much the other way as well if that phone wasn't waterproof and didn't work," Kirk said.

"He wouldn't have been able to tell people where he was and then the challenge would have been to find him when the alarm was raised."

Kirk thinks it is very concerning that the age of those who usually have safety problems is not changing.

"Those younger people that we're not really have any issues with safety-wise are then moving into that risk area - so the age isn't really changing for us."

Wearing a life jacket is down to attitude, Kirk thinks, and said it's sad that many people choose not to wear one.