Apple Watch Series 2 has been updated with new seals and adhesives developed to stop water getting in, so it can be worn in the water at depths up to 50 metres.
"It's been completely reengineered," said Apple chief operating officer Jeff Williams. "You can wear it whether you're swimming or surfing."
The most significant change to the design is the speaker, which has been configured to eject any water that enters.
The "swim-proof" model has been tested with machines to prove it can withstand constant use in the pool or sea.
Algorithms have been developed to accurately calculate calories burned while swimming, based on an individual's stroke style.
Other updates include a dual-core processor, a display that is two times brighter and screen graphics that are twice as sharp.
It will be available in a wide variety of styles and colours, with casings including aluminium, stainless steel and a new ceramic edition – which Apple claims is four times harder than stainless steel.
Apple has continued its collaboration with French fashion house Hermès to release new handcrafted leather straps for Series 2.
A partnership with sports brand Nike has resulted in the Apple Watch Nike+, marketed as a simple alternative to more complex GPS run-tracking devices.
As with the iPhone 7 and iPhone 7 Plus – also revealed at today's Apple Keynote presentation in San Francisco – the Apple Watch Series 2 will be available to pre-order from 9 September 2016. Prices will start from $369.
The original Apple Watch, now called Series 1, will come with the same dual-core processor for $269.
The Apple Watch has received very mixed reactions since it launched in September 2014.
Its critics have included Apple co-founder Steven Wozniak, who was against the company's move into the jewellery world.
Design critic Alice Rawsthorn also attacked the Apple-Hermès collaboration, which she called "a three-dimensional exercise in skeuomorphia".
A report published in July 2016 showed that sales of the Apple Watch had fallen significantly as customers anticipated the release of the new model.