Trim weights (back / shoulder)
Also: ankle weights
Trim weights help you stay horizontal in the water. They sit on your ankles, wrists, or waist.
Divers use these to fix a heavy-legged posture. I often see students struggle with heavy fins because their center of gravity is too high. Small 0.5 kg weights on the ankles can change your entire profile. This is helpful when you wear thick boots or heavy wetsuits in places like Komodo.
Avoid overusing them. If you add too much weight, you will fight your buoyancy regulator all day long. Start with just 0.5 kg on each ankle because it is easier to adjust later.