What role does a dive jacket (BCD) play in diving?

A dive jacket, commonly known as a Buoyancy Control Device (BCD), plays a critical role in managing a diver's buoyancy during a dive. The correct choice highlights its function in allowing divers to adjust their buoyancy, which is essential for both ascent and descent while underwater.

When a diver wants to descend, they can alter the volume of air within the BCD, making it less buoyant by adding weight or releasing air. Conversely, when ascending, the diver can inflate the BCD, increasing buoyancy and allowing for a controlled rise to the surface. This control prevents rapid ascents, which can lead to decompression sickness, and allows divers to maintain neutral buoyancy, enabling them to hover effortlessly in the water while they explore and observe their surroundings.

The other options, while related to various aspects of diving, do not accurately describe the primary role of a BCD. It does not provide thermal insulation or increase a diver's weight; rather, it contributes to buoyancy management. Additionally, while wearing a BCD may influence swimming technique, it does not inherently enable faster swimming speeds. The focus of the BCD is firmly on buoyancy control, making it a fundamental piece of diving equipment.