The Duke of Cambridge showed off his "buff credentials" when he donned a wetsuit and went scuba diving to mark becoming president of the activity's governing body.
Wearing a mask, air cylinder, regulator, buoyancy device and fins, William swam to the bottom of an open-air pool with a group of other divers at a local authority sports centre near Covent Garden in central London.
William is a keen diver and has become the third generation of the monarchy to take the helm of the British Sub-Aqua Club (Bsac), following in the footsteps of his father, the Prince of Wales, and grandfather, the Duke of Edinburgh.
At the Oasis Sports Centre the Duke was joined by Charles who had been Bsac president for 40 years until his son took over in May.
William who has dived in Anglesey, where he used to be stationed as an RAF search and rescue pilot, and in warmer locations overseas, joked he hoped Prince George would follow the family tradition.
He told senior Bsac members and their guests: "It's a great honour to be here with my father and accept the presidency of the British Sub-Aqua Club, continuing that from my grandfather as well."
He added: "I hope that one day my son George might follow in our footsteps.
"For the moment bath time is quite painful, but I imagine as he gets a little bit older, donning a mask and snorkel might calm him down."
Charles also gave a farewell speech after being made an honorary life member of the organisation.
He said: "I'm delighted that my eldest son is taking over from me."
The Prince, who has dived to the Tudor warship Mary Rose and in the Arctic, added: "I can only assume that his interest probably stems from the fact that I threw him into the swimming pool on many occasions - instead of putting him off, it actually enthused him, so I'm thrilled that he has decided to carry on the tradition started really by my father."
He added joking: "I know my eldest son will now demonstrate his buff credentials, by leaping bodily into this very welcoming - and I didn't realise existed - swimming pool."
William's dip in the pool began with him joining a group of six youngsters who were having a snorkelling lesson in the shallow end.
In bright summer sunshine they all sat in the water with their backs against the pool wall and listened to an instructor who was taking the schoolchildren through underwater hand signals.
The group then snorkelled a short distance one after the other, with the Duke's large frame quickly covering the water after a few flicks of his fins.
When it was announced a race would be staged, William joked: "Let's race, let's do it - give me a lead start."
But he finished joint second and said to the winner "you're too fast".
Eugene Farrell, Bsac's chairman, joined William in the pool and helped him put on his buoyancy device and air cylinder and the Duke changed his snorkel for a regulator.
William swam with a diving buddy, 17-year-old Tom Billingham, from Stourbridge, West Midlands, and the pair travelled into the deep end with the rest of their group before a taking part in a circuit.
The teenager said of William afterwards: "He's a natural, a proper seasoned diver.
"He gave clear, confident signals underwater. It looks like he's got it in his blood."
The British Sub-Aqua Club hopes that their new president will help attract new, younger members.
The Duchess of Cambridge is also known to have learnt to dive, like William, with Bsac, which is made up of 120 dive centres and 1,000-plus family friendly and sociable clubs, run by volunteers, up and down the country and abroad.
It represents more than 30,000 scuba divers and snorkellers and welcomes new members from complete beginners upwards, including those who have trained with other agencies.