The man charged with keeping people safe on Northland waters has revealed his top gripes about behaviour this summer.

Northland Regional Council Harbourmaster Jim Lyle said the issues arising again and again were people diving without warning flags and boaties setting out without proper lighting.

NRC rules meant anyone diving off a boat must display a blue and white flag, no smaller than 60cm square. This signalled passing boats to slow down to 5 knots within 200 metres of the divers' vessel.

"If vessels have divers out and don't have flags, and another vessel speeds past and divers surface, the result can be quite horrific," Mr Lyle said.

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The other most common smack on the hand had been for boats without proper lighting.

Most recreational vessels needed arcing lights on each side - white on the masthead, green to starboard, red to port and a white sternlight.

"We have people taking off before dawn with no navigation lights ... If the smaller boats don't have lights a bigger launch can come along and not even see them," he said.

On top of the run-of-the-mill influx of cruise ships and Auckland yachties, staff at the harbourmaster's office had dealt with several dramatic salvage missions.

These included a concrete yacht that ran aground on Boxing Day at Ruakaka and became wedged in the sand for more than two weeks; a 7-metre yacht that crashed into the Cavalli Islands on January 5 and a fishing boat which struck rocks in the Bay of Islands and sank on January 11, sparking a search for three men on board.

The season of misbehaving boaties kicked off on November 29 last year when two men allegedly crashed a 106-year-old schooner into a sandbar in the middle of Hatea River.

Disobeying the harbourmaster could mean prosecution, resulting in prison time or fines up to $10,000.