At its peak more than 10,000 people were tuning into the live stream on You Tube.
Read more: No launch for Rocket Lab today
Blast off! Rocket Lab successfully reaches orbit on second attempt
At 4.30pm conditions aligned and the rocket was preparing for takeoff, and was in countdown, when the vessels were spotted.
The exclusion zone had been well publicised for more than a week prior to the attempted launch, Rocket Lab said.
Police confirmed they had spoken to the skipper of a yacht and no charges would be filed.
The Police Maritime Unit had assisted the Hawke's Bay Regional Council in advising the skipper to leave the zone.
"The yacht then left the area, and no further action will be taken by police," a spokesman said.
Police said they weren't aware of any other vessels in the area.