Wetsuit

Night dives and long runs into an underwater lab are when it goes on. Sea water is cold, being wet makes you colder, and the game text sums the suit up as protection from frigid waters. Nothing in that text mentions bullets, so treat it as clothing for the sea and not as armour.

You cannot craft or research it, so every suit comes from loot or a trade, and a spare one recycles into a few metal fragments. So the real question is whether a given dive needs the one you have.

Picture the plan: a boat anchored over the lab after dark, your diving tank full, a diving mask on and the Abyss Torch in hand. Without the suit, the cold starts eating your health while you are still in the first rooms, and you surface with half the crates unopened. With it, the cold stops being the reason you turn back.

For a quick grab near the shore at midday, leave it at home and save it for a night that needs it. And when the water still bites through the suit, drink an advanced warming tea before you jump.