For a long time I was under the impression that diesel subs stand out like a sore thumb on sat photos and nation states watch them all like hawks, but apparently not and drug cartels can just roll around as they please in them.
After 200 meters the ocean is practically opaque. Objects much deeper than that are reflecting a handful of photons. Below 500-1000 meters you're talking about photons per second at the surface.
Water is one of a quite small number of general radiation absorbers. To block alpha/beta/gamma radiation you need pure density- more mass per volume to slow down high energy particles. You can use heavy atoms like lead or uranium, or very densely packed lighter atoms. Neutron radiation is different- the actual number of atoms per volume is critical to maximizing the number of scattering events. That means you use things like polymers- hydrocarbons, so you have as many small atoms (hydrogen) in a volume as possible. Water is one of the denser liquids, while still being 11% hydrogen by mass, vs 14% for pure polyethylene. That makes it quite good.
Even sound isn't great underwater, relatively speaking- being a fluid in motion, there's a constantly-changing distortion on everything. The thermal conductivity also means thermal signatures spread out quickly.
Watched countries argue a bit over diesel vs nuclear submarines and this is where I've been imprinted with the idea that nuclear subs are the only actual true stealth subs you can build. I may be wrong here, but the evidence seems overwhelming based on usage. Diesel subs are for homeland defense purposes and don't worry about the enemy knowing their whereabouts, nuclear subs go silent for months and everyone is completely clueless as to where they actually are.
https://www.popularmechanics.com/military/navy-ships/a281849...
AIP technology means non-nuclear subs still have lots of uses. They can get closer in shore to gather signals intelligence or drop off/pick up SF.
[0] https://www.popularmechanics.com/military/navy-ships/a197847...