It happened at the Moelfre Lifeboat open day at 16:10 BST on Saturday, with two RNLI vessels rescuing him.
He was then taken to Ysbyty Gwynedd, Bangor, by a UK Coastguard helicopter, with his condition unknown.
North Wales Police is investigating the incident and has appealed for witnesses.
Organisers had earlier warned about safety in the water on its Facebook page, imposing a speed restriction on boats travelling to the event.
They had said: "From past experience, we are expected to have over 100 vessels.
"Some will be kayaks with children on board, water safety is very important. Many adults and children will be swimming in the bay. Keep your eyes peeled please."
The event, at the RNLI station, was to allow people to meet the crew and celebrate their life-saving work.

The county council plans to build 200 council homes and 300 affordable homes.
The first will be built on the site of the former Custom House Lane School in Connah's Quay.
Meanwhile, the nearby "old dairy" on Mold Road is being flattened along with some terraced homes on the adjacent High Street. The former dairy has stood empty for more than 30 years.
The council has asked experts to draw up proposals for the site, which will be levelled and seeded once demolition work is completed in March.

The number of corporate insolvencies fell year-on-year by 21%, to stand at 188, in the first three months of 2015.
The figure was 10% lower than the previous quarter.
The number of administrations, which typically affect larger organisations, remained virtually the same as a year ago, with 20 cases.
A quarter-on-quarter comparison showed a similar increase, with two more businesses going into administration in 2015 than the last three months in 2014.
Liquidations, which tend to affect smaller businesses, were down by 24% on a year ago, at 168.
KPMG said the figures suggested the small and medium-sized enterprise (SME) sector was in a healthier economic position as it entered the second quarter of 2015.
Blair Nimmo, head of restructuring for KPMG in Scotland, said: "It's clear as we enter the second quarter of 2015, in general, businesses are in a stronger position.
"Buoyed by an overarching recovery, economically we're seeing a positive trading environment for both large and smaller organisations in Scotland, which is reflected by the drop in insolvency appointments.
"That being said, volatility in oil prices as well as uncertainties around the outcome of the general election may have wider repercussions for the economy as businesses adopt a 'wait and see' approach to growth."

Telford and Wrekin Council believes the proposals for MoD Donnington could create up to 700 new jobs.
Donnington provides logistics support for the Army, handling military equipment and weapons.
The local authority said it was putting together a planning application for a one million sq ft (93,000 sq m) development at the site.
The council said it had been in talks with the MoD over the past 18 months to turn Donnington into one of the main support depots.
Telford and Wrekin said the plans were still at a very early stage, and depended on winning investment from the MoD.
Councillor Bill McClements, responsible for finance and enterprise, said it was a great opportunity for both the MoD and the local economy.
"It's looking good and we're very optimistic," he said.
"Potentially it safeguards over 1,000 jobs and can bring in between 500 and 700 new jobs, plus extra jobs and investment in the supply chain.
"We're removing any barriers. We're offering incentives, we're offering to train and even help to build it."
Mr McClements said the planning application was expected to be submitted before the end of the year.
In a statement, the MoD said it was too early to talk about job opportunities and that it was also in talks with a number of other councils.
The MoD said the negotiations were part of a project to rationalise its procurement and logistics operation.

6 December 2014 Last updated at 09:15 GMT
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