How does a Submarine dive Underwater

A ship floats because it displaces a volume of water equal to itself in weight.
A submarine dives and surfaces by varying its weight.
It does this by filling ballast tanks with air or water.
When diving, the sub takes on water to increase its mass in relation to the surrounding water; it sinks.
To surface, the crew discharges water using compressed air stored onboard.
The vessel lightens, and buoyancy is restored.
Divers take a similar approach to controlling floatation.
They often wear weight belts to help them sink to depths despite wetsuits and other buoyant equipment; when they want to surface, they may use a special vest with bladders that can be filled with compressed gas to increase buoyancy.