Depth-based cold hazard

Currently there is no cold hazard in deep water.

In the real ocean, temperature drops with depth. After the thermocline at ∼200m, water becomes very cold, and at 1000m it is around 4°C.

In Subnautica 2 Early Access, the deepest biome is Root Canyon at approximately 760 meters, but there is no cold effect there.

My suggestion is to add a gradual cold system: the deeper you go, the colder it gets. It should not be instant damage, but a progressive effect - first visual shivering/breath condensation, then slow health drain, and finally hypothermia if you stay too long without a Thermal Suit or without returning to a heated vehicle. This would make exploration much more realistic and immersive.