Seeing as the Torah has just informed us that all living things died, why does it add that HaSh-m "blotted them out"?
Why does this verse record that man was "blotted out " first, and only then the animals?
Maharal (Nesivos Olam, Nesiv ha'Shetikah Ch. 1, p. 101): That which possesses 'Tzurah" is ready to achieve its function, but can also be easily ruined. This is what happened to man at the Mabul. 1 Also refer to 7:22:1.1:2.
"Tzurah" means purpose and its actualization; and man is the purpose of creation. See also Maharal, Netzach Yisrael (Ch. 39, p. 167) on this topic. (EK)
Why does it repeat later in this same Pasuk, "and they were blotted out"?
Ramban #1: Not only did every single creature in the world perish, but not even a bird's egg or that of a Sheretz remained in a tree or in the ground.
Ramban #2 (citing Chazal): To teach us that they perished both from this world and from the World to Come.
Bearing in mind that the word "Ach" always comes to exclude, why does the Torah write, "va'Yisha'er Ach Noach"?
Rashi #1 and Ramban: The simple explanation is that Noach alone remained, while the entire world perished. To stress that every single human-being, animal, beast, bird and Sheretz perished, and that besides Noach and his family, not a single one remained (Ramban).
Rashi #2: Chazal however, derive from here that a little bit of Noach was missing - the strength that ebbed from him, as he tended to his gigantic flock of animals and beasts. 1 He sighed from the effort.
Rashi #3: Some explain that a lion struck him for being late with its food. 2
Da'as Zekenim, Rosh, Ba'al ha'Turim: From the double exclusion - (a) va'Yisha'er Noach; (b) Ach 3 - we learn that Og too, survived the Flood. 4 Also refer to 7:23:3.3:1; refer to 7:23:4.
Chochmah u'Musar (Vol. 2, p. 300): Why didn't Noach give this work to his sons? Even Cham was a Tzadik, for he was saved! Rather, he understood that HaSh-m wanted him (alone) to serve them, for He said (Bereishis 7:2) "Tikach Lecha." (This is unlike Sanhedrin 108b, which says that Noach and his sons fed the animals day and night.)
Chochmah u'Musar (Vol. 2, p. 302): It says (Bereishis 9:2) that animals will fear man! (Noach was struck) because he was lacking complete fulfillment of, "just like He is merciful, so must you be;" for HaSh-m anticipates each creature's changing needs.
A double exclusion always comes to include. Ba'al ha'Turim notes on our Pasuk that "Ach Noach" (only Noach) equals "Og" (spelled with a Vov) in Gematriya (79).
Rosh: Sichon was Og's brother. How did he survive? Pairs were saved; also Og's mother was saved. She was pregnant with Sichon; alternatively, afterwards Og fathered Sichon through her.
"Only Noach and those with him on the Ark remained." However, Rashi (to Bereishis 14:13) cites a Midrash that the giant Og was also spared?
Gur Aryeh (14:13): According to the Gemara (Zevachim 113b), Og was inside (or beside) the Ark. He is not mentioned explicitly 1 because he and all of his descendants were later wiped out, by Moshe (Bamidbar 21:25).
Why was Og spared from the Flood?
Targum Yonasan (to Bereishis 14:13): So that the world should witness the power of HaSh-m.
Gur Aryeh #1 (to 14:13): So that Moshe would later be able to kill him (demonstrating HaSh-m's kindness to Moshe and to Bnei Yisrael).
Gur Aryeh #2 (to 14:13): Og was descended from Shamchazai, an angel who had descended from Heaven (see Nidah 61a). Because he had a G-dly aspect, it was fitting that Moshe, man of G-d (Devarim 33:1), should exact punishment upon Og, and not by mere water. 1
Water is corporeal, it is material with no form (Tzurah). In contrast, Moshe was "drawn from the water" (Shemos 2:10), meaning his Tzurah was complete and removed from any material aspect. See Maharal (Gevuros Hashem, Ch. 18 p. 82).
If the water was boiling, how did the fish survive?
Ramban #1: They fled to the depths of the ocean, which was not heated by the boiling water which flowed into it.
Ramban #2: Only the fish in the sea survived. 1
After the Flood, the fish swam from there (back) to the rivers. In any event, the fish must have survived, seeing as they did not enter the Ark with all the other animals - as is evident later (see Bereishis 9:9-10). See notes of R. Chavel on the Ramban.
QUESTIONS ON RASHI
Rashi writes: "va'Yimach" means, "and He wiped out [all of existence]," an active verb. How might we have misunderstood it?
Gur Aryeh: We might have thought the verb was in passive form, "and existence was wiped out," i.e. that HaSh-m brought the waters, and the creatures drowned by themselves. Rather, it means that HaSh-m actively wiped them out.
Rashi writes: ... In the homiletic Midrash, we learn that Noach became physically debilitated. Why does Rashi choose to cite this Midrash as well?
Gur Aryeh: The word "Ach" would be unnecessary for its literal meaning, "only;" for surely only Noach was left.
Rashi writes: "... A lion struck him." How do we know that it was specifically a lion?
Gur Aryeh: Man is supposed to rule over all animals (1:26), and Noach was a complete Tzadik. HaSh-m would not have allowed any lesser creature to attack him; it must have been the lion.