Casio has put a heart rate monitor into a G-SHOCK watch for the first time in its G-LIDE line, a family of timepieces built for sport on and beside the water. The GBX-H5600 is the first G-LIDE model with the sensor; the line has long carried a tide graph and moon data for surfers, and G-LIDE sits within G-SHOCK, which dates to 1983.

The watch uses an optical sensor to follow heart rate during exercise, and Casio says it records distance, calories and other measures, with training modes for running, walking, gym work and interval training. Readings are meant to be reviewed in the CASIO WATCHES phone app, and the display is a memory-in-pixel LCD chosen for legibility. The release does not give a price or say which markets the watch will reach first.

When a tide watch becomes a tracker

A G-LIDE was designed to answer one question, what the water is doing, and a watch that also counts heartbeats is a different kind of object. Casio is following its customers: the people who paddle out at dawn also train, and the watch industry has learned over a decade that a sensor producing a number will be used whether or not it was bought for that purpose.

What has not changed is the arrangement underneath. The training data lives in one company's app, a small trade of privacy for convenience that buyers rarely weigh, and the readings are for reference rather than medical use. A heart rate sensor for sport and a clinical measurement are not the same claim, and this watch makes only the first. The GBX-H5600's specifications are Casio's own. Whether a surfer needs a heart rate monitor, or simply likes having one, is a question the launch leaves open.