What is "ki'Nechalim Nitayu" referring to?
Rashi: 'Like rivers that flow long distances'.
Seforno: The Shuls and the Batei-Midrash for the multitude of Yisrael are comparable to rivers that are planted next to fields to water them, And the Torah-leaders draw from those who study to teach the masses; Similarly, those who Daven are comparable to gardens that are grown beside the river, which do not cease to provide fruit. 1
Targum Onkelos: 'Like the fertile garden on the banks of the River P'ras'.
Targum Yonasan: 'Like self-generating rivers are B'nei Yisrael who congregate to study Torah'.
Seforno: As the Gemara states in Rosh Hashanah, 17b
And what was Bil'am referring to when he said "ke'Ganos alei Nahar"?
Targum Yonasan: 'And like gardens that are planted on pools of water are their Talmidim who study Torah in groups in their Batei-Midrash' and whose faces shine like the sky that Hashem created on the second day of the creation and stretched in honor of the Shechinah'.
What is "ka'Ahalim Nata Hashem" referring to?
Rashi #1 (citing Targum Onkelos) and Rashbam: 'Like the aloes (a spice) that Hashem planted in Gan Eden'. 1
Rashi #2: 'Like the heavens that Hashem stretched like a tent. 2
Ramban (on Pasuk 7): The current Pesukim are praising Eretz Yisrael, comparing it to a fertile garden and to a spring of water that will not dry up.
Why did Bil'am compare Yisrael to a cedar-tree beside water?
Targum Yonasan: Just as the cedar is taller than all other trees, so too do Yisrael, who study Torah, rise above all the other nations.
Just as a cedar-tree thrives next to water, so too, do Yisrael thrive when they study Torah.