More children may drown without better school swimming provision, the Amateur Swimming Association (ASA) has warned.
Half of seven- to 11-year-olds in England, some 1.1 million children, cannot swim the length of a standard pool, research suggests.
Only 2% of schools surveyed delivered the government's recommended 22 hours a year of swimming lessons.
The report says Ofsted should focus PE inspections on swimming as "it is the only sport that can save lives".
Some 407 people died from drowning in 2011, according to the most recent figures from the National Water Safety Forum. Of these, 47 were aged under 19.
"These numbers could increase in future if the current issues with school swimming programmes are not addressed," warns the report.
Swimming to safety
A survey of 3,501 schools, for the ASA and its sponsors Kellogg's, found that 51% of seven- to 11-year-olds could not swim 25m (82ft), the length of a standard pool.
The national curriculum for schools in England says children should be able to swim at least 25m unaided by the end of primary school.
Some 40% of 1,000 parents of 10- and 11-year-olds surveyed for this year's report said their child could not swim 25m.
Some 52% doubted whether their child could swim to safety in open water, 39% said their children were not having swimming lessons and one in 10 said their child only swam on holiday.
The report suggests state school pupils spend an average of just eight-and-a-quarter hours a year in school swimming lessons - well under the 22 hours required by the national curriculum.
The high cost of swimming lessons, the price of transport and lack of time in school day all impacted negatively on school swimming provision, says the report.
The authors say swimming is in danger of being "sidelined" because of a lack of monitoring by Ofsted, increased pressure on schools to deliver good exam results and squeezed budgets.
The authors want the government to instruct Ofsted to include swimming provision in school inspection reports.
They also call for PE funding to be focused on swimming. They want the lion's share of the £150m the government will put into primary school sport from September to be spent on swimming.
David Sparkes, of the ASA, said: "Swimming is one of the easiest, safest forms of exercise for children of all abilities, and school swimming is the single most effective way of teaching children how to be safe in and around water.
"Yet swimming is one of the few areas of a child's statutory education that is all too often left unmeasured, unchecked or, for 1.1 million children, unfulfilled.
"The additional £150m of ring-fenced investment by government for PE and school sport this September can provide a real lifeline for school swimming.
"Schools have a rare opportunity to seize the moment in September 2013, and take action by investing in an activity that has a lifelong legacy and the ability to keep future generations safe."
David Walker, of the Royal Society for the Prevention of Accidents (Rospa), said: "Having the swimming and water-safety skills to save yourself or others doesn't come instinctively, it has to be taught.
"Rospa hears all too often how parents believed their children could swim, only to find out their abilities were little more than being able to float and doggy paddle."
A Department for Education spokeswoman said swimming would remain in the national curriculum.
She added: "We are giving £150m directly to primary schools to spend on improving PE. They can use this money for anything to improve sport, from buying in professional coaching to covering the cost of getting children to and from the swimming pool.
"We have also invested £750,000 to develop an initial teacher-training pilot to improve the standard of primary PE specialists, with the first 120 [specialists] ready to join schools from September."