Soaring weekend temperatures attracted hundreds of people to the riverside at Abbey Meadow in Abingdon.
But with the swimming pool closed, toddlers and youngsters crowded into the the water feature while some older children and youths swam in the Thames.
And that has prompted another warning about the risks attached to river swimming by members of Stop, Save The Outdoor Pool, who have until the end of July to come up with a viable long-term business plan to keep the pool.
The Vale of White Horse District Council wants to remove the 55-year-old pool because it says it is too expensive to maintain and run.
The council wants to replace the pool from next summer with a £100,000 family water feature.
But it has offered to lease the pool to Stop at a peppercorn rent if the campaigners can come up with an acceptable business plan.
Stop campaigner Paul Gustafson said: "We have warned the council of the dangers of children swimming in the river because the pool is closed.
"There are river-borne diseases from which people could suffer and there are boats, underwater hazards and currents."
Another campaigner Peter Harbour said: "One person is drowned in the Thames every year for every five miles of the river. Sadly one teenager was drowned in Sunbury during the weekend heatwave.
"If a drowning occurs in the Abingdon area, who will tell the bereaved parents? We have a heated pool which should be open."
One of a group of six young soldiers swimming in the river said: "There have been hundreds of families and kids down here enjoying the water feature and relaxing by the river. The pool is empty so the river is the best place to cool down in. You could be at risk from diseases, I suppose, but I've had enough jabs for when we serve abroad to protect me."
The leader of the Vale council Jerry Patterson said: "If the hot weather could be guaranteed every summer then there would be no problem with keeping the pool but we can't and it's too expensive. Young children enjoy the water feature and families relax in Abbey Meadow. It's risky using the river and people need to act res- ponsibly."
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