From a scuba diving / apnea (free) diving perspective knowing your barometric pressure can let you know your depth - At 1 bar you are at surface level, with each 10m descent you are at a further 1bar. So at 10m you're at 2bar. At 15m you're at 2.5 Bar etc.
There are many risks with being underwater from a scuba perspective which increase with depth. From as little a depth as 5m you can become (depending on you as an individual) susceptible to the effects of nitrogen - as you breathe in and out under water your body takes in more nitrogen than you do at the surface, this again varies on you as an individual (fat absorbs more nitrogen etc). Apnea divers tend to not be affected as they hold a continuous breath. Nitrogen narcosis can cause a lot of problems, from becoming incredibly slow in responding to hallucinating.
Also with scuba you must continue to breathe, holding your breath can lead to a lot of issues. With depth, and therefore pressure increases, your lungs volume changes, so if you imagine a balloon full of air at the surface, at 40m the balloon will be 1/4 the size but hold the same amount of air. Now imagine you fill that balloon further whilst under water, and then coming up - the balloon will burst as the air expands coming up. This is exactly why divers can never hold their breath, and why you may get a poke in the stomach from your buddy if they don't see bubbles.
This is commonly recorded on your dive computer though, but some people dive with a watch as well, as a fail-safe.