One of the roles of dichotomies is to explain the origins of the universe in by definition in contrast – to explain that they stand opposite of each other, yet are defined by one another. As a book that governs religious morals, this is analogous of the right-wrong, good-bad dichotomies that are prevalent throughout. The book of Genesis states that God introduced light to darkness which turned into day and night respectively. God created land, turning the waters into “seas”. God created male and female, heavens and earth.
As effective as they are, these dichotomies are also a poetic, aesthetically pleasurable method of definition. Instead of listing the countless things that encompasses definition of “land”, we are able to infer its meaning by the simple statement of what it is not – the “sea”.
These dichotomies are also given a positive association by usually by a following statement that God found them to be good. In the later books of Genesis, we find the introduction of “bad and evil” so all these statements of what God saw that was “good” is part of another dichotomy, and it perhaps its most prevailing – the dichotomy of good and bad.
You have a good insight on the dichotomy of the land and the sea. It makes you realize that to describe something, one does not always have to lay down the specific meanings and definitions of that certain word. Sometimes, people will just get a better understanding of that single word by knowing what its not.