The two-word answer is: Very Carefully. First, I use the Garmin-Dexcom integration, which is Dexcom's officially-approved way to view blood sugars on smartwatches. They partnered with Garmin on a joint release and promotion of the integration, which is a significant step up from third-party apps like xDrip and Nightscout that require GitHub downloads, builds, and connecting up multiple apps and servers by hand. In my experience, I've found that the integration works best when the Dexcom CGM transmitter is near, at, or above the surface of the water and I'm using a normal swim stroke that gets the watch out of the water enough to receive a reading. Since I swim in open water and always use a visibility buoy with a built-in sealable dry sack, I put my phone in a drybag inside the dry storage in the buoy. If the system doesn't pick up a reading, it typically buffers it on the transmitter until it can send one to the phone, which shares it with the watch. My other option--and this is just a common-sense backup strategy, is to train consistently so I know what my blood sugar is going into the water and at each fueling stop, typically around a mile. That's when I usually take in 50g of carbs from either two Untapped maple syrup packets tucked into my jammers or wetsuit, OR have my kayaker toss me a feed bottle with that same amount dissolved in water. It's hard to tell in cold water if you are sweating or feel shaky, so I look for clues like a sudden degradation in my swimming technique, clenched jaw, or a feeling of doom and gloom; and when in doubt, I get carbs.