Watch on Board Ship
There are two sorts of watch- the long watch of four hours, and the dog watch of two, from 4 to 6; but strictly speaking a watch means four hours. The dog watches are introduced to prevent one party always keeping watch at the same time. (See Wolf, Between dog and wolf, Dog-Watch.)
There are two divisions which perform duty alternately- the starboard watch and the port watch. The former is called the captain's watch in the merchant service, often under the command of the second mate; the port watch is under the command of the first mate.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894