How can a submarine choose to sink or float? If a boat in a canal lock sank, would the water level in the lock rise or fall? (not sure what a lock is? see below)

Answer: The choice of a submarine to either sink or float depends on the captain of that submarine. For a submarine to sink the captain just have to allow water to fill the water tank (lock), making the submarine more dense than the water it has displaced. For it to float, it just have to empty the water tank and allow air to fill it.

When a boat in a canal lock sank, the water level will rise, because the volume of the boat has displaced more water, and this displaced water will cause the water level to rise. The space occupied by an object in a water, is equal to the volume of water been displaced.