Building on Josh61's answer, the phrase "dry land" as an idiomatic expression may have come from the book of Genesis (1:9-10) (as well as other references throughout the original Hebrew).
King James Bible (1611)
(9) And God said, Let the waters under the heaven be gathered together unto one place, and let the dry land appear: and it was so. (10) And God called the dry land Earth; and the gathering together of the waters called he Seas: and God saw that it was good.
Wycliffe's Bible (late 1300's)
(9) Forsothe God seide, The watris, that ben vndur heuene, be gaderid in to o place, and a drie place appere; and it was doon so. (10) And God clepide the drie place, erthe; and he clepide the gadryngis togidere of watris, the sees. And God seiy that it was good;
Septuagint (3rd century BCE)
(9) Καὶ εἶπεν ὁ Θεός· συναχθήτω τὸ ὕδωρ τὸ ὑποκάτω τοῦ οὐρανοῦ εἰς συναγωγὴν μίαν, καὶ ὀφθήτω ἡ ξηρά. καὶ ἐγένετο οὕτως. καὶ συνήχθη τὸ ὕδωρ τὸ ὑποκάτω τοῦ οὐρανοῦ εἰς τὰς συναγωγὰς αὐτῶν, καὶ ὤφθη ἡ ξηρά. (10) καὶ ἐκάλεσεν ὁ Θεὸς τὴν ξηρὰν γῆν καὶ τὰ συστήματα τῶν ὑδάτων ἐκάλεσε θαλάσσας. καὶ εἶδεν ὁ Θεός, ὅτι καλόν.
(9) And God said, Let the water which is under the heaven be collected into one place, and let the dry land appear, and it was so. And the water which was under the heaven was collected into its places, and the dry land appeared. (10) And God called the dry land Earth, and the gatherings of the waters he called Seas, and God saw that it was good.
Original Hebrew (vocab list)
(9) וַיֹּאמֶר אֱלֹהִים, יִקָּווּ הַמַּיִם מִתַּחַת הַשָּׁמַיִם אֶל-מָקוֹם אֶחָד, וְתֵרָאֶה, הַיַּבָּשָׁה; וַיְהִי-כֵן.
(10) וַיִּקְרָא אֱלֹהִים לַיַּבָּשָׁה אֶרֶץ, וּלְמִקְוֵה הַמַּיִם קָרָא יַמִּים; וַיַּרְא אֱלֹהִים, כִּי-טוֹב
(9) And God said: 'Let the waters under the heaven be gathered together unto one place, and let the dry land appear.' And it was so. (10) And God called the dry land Earth, and the gathering together of the waters called He Seas; and God saw that it was good.
A distinction is made here between יַּבָּשָׁה, the word that's translated to Greek as ξηρά and then to English as "dry land", and אֶרֶץ / γῆν, which is the typical word for "ground, land, Earth, ...".
As far as I can tell, the former is only used in biblical texts, whereas the latter is part of modern Hebrew. Also, According to google translate, ξηρά literally means "the dry", rendering the first part of verse 10 "And God called the dry, 'Land'."
Edit: As @oranja corrected me, יַּבָּשָׁה is used in modern Hebrew, although mostly as the translation of continent rather than the biblical "place that is not water".
Some other uses of the word יַּבָּשָׁה include Exodus 14:29 ("But the children of Israel walked upon dry land in the midst of the sea; and the waters were a wall unto them on their right hand, and on their left."), Joshua 4:22 ("Then ye shall let your children know, saying , Israel came over this Jordan on dry land.", and Jonah 2:10 ("Then the LORD commanded the fish, and it vomited Jonah up onto the dry land."), among others. In all cases, it's used to contrast with the waters of the sea (or river).
Thus the phrase could very well have been in use as an idiomatic expression long before the first attestation to it in the 13th century origin that Etymology Online cites.