Used in tandem with a lightweight inflatable snorkeling vest, this combination adds both versatility and safety. Vests are also a logical place to attach a small but powerful whistle, just in case you ever need to attract some attention. Here, take a look at this nifty little capillary depth gauge. Just wear it like a watch. I hadn't seen one of these in years, a simple flat disk holding a circular clear plastic tube.
Water enters the tube and shows the depth.
No moving parts, yet very accurate in the shallows and virtually indestructible. Perfect for snorkelers and inexpensive. Phil was on a roll. You can easily fasten anything on to your wrist with these. I asked Phil for a few examples.