sailors, fishermen, soldiers, hunters and associated with the realia, phenomena and conditions of their occupations. In Smith's classification we also find groups of idioms associated with domestic and wild animals and birds, agriculture and cooking. There are also numerous idioms drawn from sports, arts, etc.
This principle of classification is sometimes called "etymological". The term does not seem appropriate since we usually mean something different when we speak of the etymology of a word or word-group: whether the word (or word-group) is native or borrowed, and, if the latter, what is the source of borrowing. It is true that Smith makes a special study of idioms borrowed from other languages, but that is only a relatively small part of his classification system. The general principle is not etymological.
Smith points out that word-groups associated with the sea and the life of seamen are especially numerous in English vocabulary. Most of them have long since developed metaphorical meanings which have no longer any association with the sea or sailors. Here are some examples.
To be all at sea — to be unable to understand; to be in a state of ignorance or bewilderment about something (e. g. How can I be a judge in a situation in which I am all at sea? I'm afraid I'm all at sea in this problem). V. H. Collins remarks that the metaphor is that of a boat tossed about, out of control, with its occupants not knowing where they are. 
To sink or swim — to fail or succeed (e. g. It is a case of sink or swim. All depends on his own effort.)
In deep water — in trouble or danger.
In low water, on the rocks — in strained financial circumstances.
To be in the same boat with somebody — to be in a situation in which people share the same difficulties and dangers (e. g. I don't like you much, but seeing that we're in the same boat I'll back you all I can). The