I remember (barely) from a diver friend that colors disappear below 20 meters (everything appears blue, unless a bright light is shone), and sun light disappear

I remember (barely) from a diver friend that colors disappear below 20 meters (everything appears blue, unless a bright light is shone), and sun light disappears completely below 70 meters. You'd assume that an optical system with flood lights could theoretically work, but then it would be pretty simple to monitor for light at these depths (for counter-intelligence/retaliatory purposes)