Timekeeping might actually not be the most practical feature a rugged adventure watch can offer: In some situations, knowing your altitude or depth underwater might be as useful or even more critical than timing. They're rare and a bit exotic, but watches with altimeters and depth gauges make a lot of sense if you're actually doing the things many sport watches were designed for.
It's even more fascinating when these features are purely mechanical — and when you realize that they first appeared on watches back in the 1960s, back when they might have actually been relied upon. Today, there are all kinds of gadgets and tech that do the same jobs, but sometimes it's nice if this all just fits on your wrist. You can get advanced digital watches with altimeters and depth gauges, but there are also watchmakers offering these features in old-school mechanical watches with Swiss craftsmanship.
Altimeters and depth gauges indicate altitude and depth, respectively, but they're both actually using atmospheric pressure translated into feet or meters. There are different ways of measuring pressure, but watches usually use capsules, diaphragms, bellows or something called Bourdon tubes which flex, contract or otherwise change shape in response to pressure. This principle works in air and in water, but altimeters and depth gauges each have their own specific considerations.
Let's start at sea level.
Depth Gauge Watches
Depth gauge watches measure your depth underwater, so you'll naturally find them on dive watches. Such watches were once made by major companies like Girard-Perregaux, Jaeger-LeCoultre, IWC, Blancpain, Panerai, Bell & Ross and others (even Timex!), but today they're rare. There are only a couple notable companies that currently offer mechanical depth gauge watches: Oris and Favre Leuba (and they're the same companies that offer mechanical altimeter watches).
As is the case with other features, watchmakers have developed different solutions to the incorporation of depth gauges. In any case, one needs a method of registering water pressure outside the watch and a mechanism inside the watch that translates it into a display — and the watch also has to remain water-tight the whole time. (Further, depth gauge watches will sometimes indicate your current depth as well as record the deepest you've gone on the current dive.)
Before starting a dive you'll often need to "activate" the depth meter, resetting the maximum depth reading as well as calibrating the gauge so you get an accurate reading while underwater. Just don't forget to make sure all crowns are fully screwed in again and your watch is nice and water-resistant before jumping in. Digital watches also use similar ("aneroid") sensors and also need regular recalibration.
Altimeter watches are traditionally associated with aviation, but commercial pilots today have much more sophisticated technology for measuring all the things that watches used to be used for (and more), so they don't find much use in altimeter watches — and passengers will also be disappointed that their altimeter-equipped watches won't be of much use since cabins are pressurized, of course. One might come in handy if you're flying small aircraft, but altimeters are perhaps most useful today for activities involving mountains like skiing or hiking.
Altimeters are historically even more rare in watches than depth gauges, but today they're more common thanks to brands like Casio offering them in digital form and within affordable packages. Just like with depth gauges, altimeters will often need calibration in both mechanical and digital form. Altimeter watches are technically interesting for watch nerds, but for people in mountainous regions or with adventurous lifestyles they also might prove useful on a day-to-day basis.