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 Pasuk. 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.
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 4 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.
That appears in the valleys and in the mountains.
Why is "Eretz" mentioned seven times here (from verse 7 through verse 10)?
Da'as Zekenim, Hadar Zekenim, Rosh: They correspond to the seven nations.
Seforno: [It says Eretz Tovah twice, and another] five times it says Eretz, for the Torah discusses five advantages that Eretz Yisrael has over other countries (refer to 8:7:2:2).