Naval Exercise

The barrel of the naval gun extends.... and a man falls into the water. Sailors dive in after him. Others run out long the spar and look down. Still others lower a boat into the water.

It doesn't say what exercise they were doing her,e but I think it likely they were practicing rescues, in case anyone fell into the briny. It seems odd to think of sailors who can't swim, bu swimming was not a requirement, or even necessary, unless you fell into the water. So mst of them tried to avoid it.