Abarim Publications' online Biblical Hebrew Dictionary
There's no real proof that the root ימם (ymm) ever existed, apart from the occurrence of one very common word that looks it was derived from it: the masculine noun ים (yam), meaning sea. This word is used for any sea or lake such as the Mediterranean (Numbers 34:6), the sea of Joppa (which is also the Mediterranean; Ezra 3:7) and the Salt Sea (Numbers 34:3) but also for any large body of water, such as the bronze sea of Solomon's temple (1 Kings 7:23). Our word in plural, ימים (yamim) denotes the general concept of seas or the whole of all the seas (Genesis 1:10).
In seventy of the almost four-hundred Biblical occurrences of this word, our word denotes a point of compass or direction, namely west (Numbers 2:18, Joshua 8:9, Exodus 27:12). It stands to reason that this interpretation of the noun ים (yam) originally meant "sea-ward". The word for east also means "past" and seems reasonable to assume that to the Hebrews the word ים (yam) had the connotation of "future" or "destination" or even "future knowledge". Please read our article on the noun ארץ ('eres), meaning land or earth, for a further look at this.
A second word that looks like it was derived from a root ימם (ymm) — and is therefore assumed to be related to ים (yam) — is the masculine noun ימם (yemim). It occurs only once, in Genesis 36:24, and what it means exactly is unknown. All we know is that the ימם (ymm) were found by Anah in the wilderness, and most translations choose for "hot springs".
Perhaps in order to express some deeper consistency but perhaps by accident, our word ים (yam) is the opposite of מי (may), which is the not-really existing singular of the very common plural word מים (mayim), meaning waters (which in turn is identical to the particle of inquiry מי (mi), meaning 'who?').
Also note the similarity between our word ים (yam), meaning sea, and יום ( yom), meaning day. Both these words' plurals are spelled ימים, meaning that the Hebrew words for 'days' and 'seas' are identical.