The entire point of a dive light is for it to withstand high pressures without water ingress.
How do you think a piece of plastic will do??
The amount of pressure on it will compress it even at shallow depths.
I get your point about plastic, especially associate with cheap + dive light, but it’s not a complete truth. Several major manufacturers of top end dive lights (Underwater Kinetics, Princetontec to name a couple) have done a remarkable business of making good, reliable lights out of plastic. I used UK dive lights as my primary for over a decade. My backup for the last 6-7 years has been a UK. That means I trust it to be there when and if I need it.
Some of the horrible dive lights coming out of China are metal, aluminum usually. So…metal is not necessarily the answer.
The real issue is the quality of the light in the first place. It’s a challenge to be super cheap, AND have reasonable function, AND still be operationally reliable. After that it’s the support of the manufacturer.
Very old topic, but still relevant IMHO.
An all plastic housing does not necessarily mean pool use only, but yes there are some real cheapies that wouldn’t last long below 10 feet. When I used to dive before LED light technology was good enough and affordable enough to use, all lights I’d see in use had plastic bodies, but made of thick ABS.
I suggest visiting a SCUBA forum, in their equipment sub section. Lights would definitely be a discussed topic with recommendations.
https://us.gearbest.com/led-flashlights/pp_696277.html is the cats ass!