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 mountains.
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).