Coastguard officials and police are urging boaties to invest in a cheap form of insurance, a plastic bag, which may save their boat and their lives.

After two or three days of fine weather the Auckland Coastguard was called to 37 breakdowns in Auckland's Waitemata Harbour and the Hauraki Gulf yesterday,

Most were for flat batteries and minor breakdowns but Coastguard duty officer Mark Leevers said boaties should never go on the water without two forms of communications, including a very high frequency (VHF) radio and a cellphone as a back up.

Mr Leevers said putting a cellphone in a plastic bag with a resealable top, could be a livesaver.

"If we don't know you are in trouble, we can't help you."

The Coastguard has long advised boaties to have a VHF radio on board tuned to the emergency channel 16, saying there was always someone listening and it was the best form of communication in a emergency.

Mr Leevers said if boaties wanted to spend their time on the water relaxing and listening to a stereo powered by the boat battery, they should be sure to have a spare battery.

Many callouts were for batteries run flat by stereos and other gear plugged into a boat's electrical system when the engine was not running.

Enough lifejackets should also be carried for everyone on board and Mr Leevers said they should be worn.

It was mandatory to carry lifejackets but it was not mandatory to wear them.

"We would recommend for a vessel under seven metres you do wear your lifejacket. The majority of boating fatalities occur in vessels under seven metres. We also recommend you carry flares."