I don't dive, but I do snorkel.
First I take a fast acting hypo treat with me in a waterproof container. I'm sure one of the glucose packs should work. I take ginger candy.
Second, I am on a pump so in my case I do a temp decrease of my basal starting a half hour before I start for 2 hours. I usually snorkel for about 1 hour.
Third I start out higher, for me at about 7.2-8.9. It's actually hard to eat without taking a correction dose and hitting an exact number.
I have a Dexcom and it will not communicate underwater for me even strapped on top of my sensor. (my iphone). I can't even hold my arm out of the water to try to get a reading as it wants to communicate all the past data first before it will read. So I am contemplating wearing a LIbra sensor too as it communicates by waving the reader or phone over it. I am thinking that I can at least hold my arm out of the water for that to work. I have no idea if it will read under water, but BT seems like it really doesn't like to communicate underwater.
I do have my iphone out in the water with me in a Stash waterproof pouch, But I don't know the depth allowed, most have depth restrictions.
But like
@Fairygodmother says, more work and you drop more. A nice easy snorkel and I might drop less than 1-2 mmol. But if I hit strong currents and choppy water I have dropped 3-4 mmol. So it's better to be safe than sorry and start out with a good cushion. I am hypo aware, but not in the water??? Although I only once dropped to 3.7 which is around when I usually definitely start to feel it and I had a headache that sent me back ashore.
If anyone knows of a CGM that works when you are in the water let me know! I am searching. I think I will be trying a Libre next!