Organised by Rotary Rotorua Sunrise for the past 10 years, the event made the shift to focus on "kids swimming for kids" last year.
Before midday, swimmers had far surpassed their target of completing a marathon, doing a combined 100km, or 2000 laps.
Two of the swimmers, Emily Russell, 11, and brother Luke, 9, clocked 70 lengths between them and raised enough money to vaccinate 350 children against polio.
It was the siblings' first time taking part and both said the cause was their motivation.
"I've seen photos of children with the polio disease and I just want to help so others don't get it as well," Emily said.
"I thought because I did swimming anyway, I'd put an effort in to help."
Delana Trask, 12, made the last-minute decision on Saturday night to take part and planned to do 40 laps, with her grandfather sponsoring 50 cents per length.
She ended up doing 100 laps and was not puffed in the slightest.
One of the organisers, Tak Tothill, said they were "just amazed" by the turnout and effort put into the day.
"They've all really embraced the cause. They just keep going."
He said it looked as though they had gone past their target of $5000, which would equate to 5000 vaccines, but wouldn't know for sure until all the pledges had been collected.
Tothill said Team Taini Deane alone had completed 342 lengths and raised more than $1700.
