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.
A jet skier has been airlifted to hospital after a crash involving a speedboat during an event off Anglesey.