Misfit's latest activity tracker counts swimmers' lengths as they exercise.

It uses similar hardware to the firm's earlier Shine - which was already waterproof - but adds new algorithms to its firmware, which were developed with sportswear-maker Speedo.

Garmin, TomTom and Timex are among firms that already sell watches that can count swimming laps.

But the Speedo Shine stands out from the other "mainstream" activity trackers in offering the facility.

Industry leader Fitbit warns that its current line-up of devices are not designed to "withstand the force associated with swimming strokes," while Jawbone had originally marketed its Up3 wristband as being suitable for the pool but had to announce a U-turn after it failed to make them as water resistant as hoped.

California-based Misfit claims its own tests suggest its algorithms provide "industry-leading accuracy" at counting laps and swimming distance.

However, the Speedo Shine cannot count the individual number of strokes or identify stroke types, which some of the more expensive swimming watches can do.

"Swimming has come late to wearable tech, partly because it's not as popular an activity as running," commented Ben Wood from the tech consultancy CCS Insight.

"But this is a good brand partnership and for Misfit it's a good way to keep its product fresh and broaden its appeal to new users."

Like earlier models of the Shine, the new device can also track steps, overall calorie burn and sleep states.

Because Misfit is a private company, it does not publicly reveal its earnings.

However, the firm recently told the South China Morning Post newspaper that it had shipped more than two million trackers over the past 18 months and was outselling Fitbit in China.

The Speedo Shine will cost $80 in the US and £60 in the UK when it is launched next month.