What is "Eretz Tovah" referring to?
Targum Yonasan: The Pasuk is referring to its luscious fruit, which is also inherent in the other praises in the Pausk. 1
See Na'ar Yonasan.
What is so special about the rivers, the springs and the underground water that are found there?
Ramban: "Eretz Nachalei Mayim" - like a satiated (watered) garden that does not require irrigation, like Egypt does; 1 "Ayanos Tehomos" - springs of water 2 and underground springs 3 which appear in the valleys and on the mountains.
Seforno: Because Eretz Yisrael is a land of rivers, springs and underground wells, not canals and stagnant pools. 4
Targum Yonasan: It is referring to the clear water of its rivers, the sweet water of its springs and the fact that its underground water 5 never dries up.
See later, 11:10 & 11.
Ramban: See Melachim 1, 18:5 and Tehilim, 104:8. They are created from the wetness of the mountains, which, in turn, are formed by the rain-water that they have absorbed or from the mist that forms there mountains.
Seforno: In the current Pesukim, the Torah is discussing five advantages that Eretz Yisrael has over other countries, by each of which it inserts the word "Eretz" (all under the heading of "Eretz Tovah").
That appears in the valleys and in the mountains.