>>7710543 (gb)
>>7710604 (gb)
sauce via reverse image search:
https://www.dailymail.co.uk/femail/article-7849319/Shirtless-Barack-Obama-pictured-paddleboarding-Hawaii.html
most likely just a sharkband, not an ankle monitor.
https://www.sharkbanz.com/products/sharkbanz-2
pic1: sharkband
pic2: modern ankle monitor
looks more like the first to me..