Aqua Park opponents have vowed to fight on after the controversial attraction began its summer pilot at Cosmeston Lakes.
The inflatable water park, which was given the go ahead in May, will operate at the eastern lake until mid-September.
Vale of Glamorgan Council said revenue generated by the attraction would go towards the park's maintenance costs.
The eight-week pilot will be reviewed in the Autumn, but a campaign group is threatening legal action to stop the water park from becoming a permanent summer fixture.
A series of protests have been held in the past few weeks at both the lakes and the Senedd in Cardiff Bay - while a petiton against the development atrracted more than 7,300 signatures since the plans first emerged.
More than £2,800 has also been raised by the Save Cosmeston Lakes campaign towards an indpendent ecological study of the site.
But the Vale Action Group has also acknlowegded that the attraction was operating "more quietly than expected" with extra precautions put in place for those using it.
The Aqua Park Group said the move to Cosmeston Lakes had created 40 jobs locally - with 16 new lifeguards being trained for the launch.
Campaign organiser Saar Lenaerts said: "We are obviously disappointed the Aqua Park has opened for business on a trial period despite so much local opposition, and without any consultation. But we do not intend giving up our fight and there is a lot going on behind the scenes."
"We are pleased the Aqua Park is operating more quietly than expected and some extra precautions have been put in place for users. However, we still have grave concerns over the water quality and will continue to monitor testing."
The Aqua Park attraction was previously based at Cardiff Bay, but concerns over water quality there led to the operators moving to Cosmeston Lakes.
The inflatable assault course, which includes over 40 connected obstacles, slides and splash zones, opened last Saturday and will operate until 14th September.
The operators also run two more sites at Lakeside in Essex and at Rutland.
Connor James, marketing manager at Cosmeston Lakes Aqua Park, said: "We are very proud of our staff and lifeguards at all our Aqua Parks. In fact, in the last five years, we've trained 400 people with RLSS UK Open Water Lifeguard qualifications across all of our sites, and 116 in south Wales."
"For this 2025 season alone, 16 new lifeguards have been specially trained for the launch at Cosemeston Lakes, so we are offering safe, seasonal fun, and new jobs and opportunities for local people."
Both Aqua Park and Vale of Glamorgan Council have insisted the attraction will have no impact on wildlife in a designated site of special scientific interest (SSSI).
Deputy council leader Bronwen Brooks said the local authority was looking to expand Cosmeston Lakes' appeal and "encourage visitors from a different demographic."
She added: "Income generated from this venture will play an important part in helping to meet the park's maintenance costs. After the eight-week pilot, the council will consider the best way forward."
The Vale Action Group says it never had a problem with the Aqua Park itself - and that their argument was purely over where it its situated.
But Saar Lenaerts added: "Our focus now is on taking legal action to ensure the Aqua Park does not become a permanent summer fixture at Cosmeston Lakes Country Park."
"We ask for the continued support of the public. We require more crowd funding for our legal battle, and we will want our petition to surpass the 10,000 signature line."
"We will continue to listen to and represent the voices of thousands of people who oppose the Aqua Park and also be the voice of the wildlife at this beautiful public park, a designated nature reserve."
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