Why is the word "Vayikachuhu" missing a 'Vav' (after the 'Ches')?
Ba'al ha'Turim: Because it was only Shimon who actually took him and threw him into the pit. 1
Why did the brothers throw Yosef into the pit?
Ramban and Seforno: Refer to 37:22:3:1,3.
Oznayim la'Torah: They actually accepted Reuven's suggestion, and threw Yosef into the pit in the interim until they decided what to do with him. 1
Ha'Emek Davar: They did not push him in from nearby, for this could break his leg! Rather, they lifted him and lowered him until the floor of the pit.
Oznayim la'Torah: Since leaving him in the pit from which he could not extricate himself would have constituted murder - See Sanhedrin, 77 - and they had decided not to murder him. This is also implied in Pasuk 28.
Having said that the pit was empty, why does the Torah add that it contained no water?
Ramban #1 (on Pasuk 22) and Rashbam: The Torah tells us that the pit was not full of water, because if it had been, having decided not to kill him directly, they would not have thrown him into it to drown. 4
Ramban #2: To emphasize that there was not even a small amount of water in it. 5
Ha'Emek Davar: It was empty of rocks, on which his head could get bashed, and of water in which he would drown. 6 That would be like overtly killing him!
Divrei Eliyahu: Mayim alludes to Torah. One who has no water (Torah) is prone to have snakes and scorpions, i.e. despicable Midos like snakes and scorpions.
Da'as Zekenim, Hadar Zekenim: Two exclusions always come to include. Do not say that it contained rocks, for 'Rak" implies something that sometimes it is Misroken (emptied) from it. Hadar Zekenim - i.e. it hides in cracks. Alternatively, we read "Yashvu Le'echol Lechem" like 'Yesh bo Le'echol Lechem' (there is something inside that can eat; Rosh - there is something edible inside). Moshav Zekenim - or, we learn a Gezeirah Shavah "Mayim-Mayim" - "Nachash Saraf v'Akrav v'Tzima'on Asher Ein Mayim" (Devarim 8:15).
See also Ba'al ha'Turim. Ramban: The brothers however, were unaware of this (either due to the depth of the pit or because the snakes were hidden in the crevices in the side of the pit). Had they known , they would have realized, when Yosef emerged alive from the pit, that he was a great Tzadik, and they would never have gone on to sell him into slavery (See Daniel, 6:23). In any event, based on their decision not to kill him directly, they would not have thrown him into a pit with snakes and scorpions. Refer to 37:24:2:2). Moshav Zekenim, Riva (22) - Reuven certainly did not know about the snakes and scorpions, since the Torah testifies that he intended to save Yosef! Alternatively, he knew; even so, this is better than overtly killing him, for perhaps Yosef is a snake-charmer and can save himself. Moshav Zekeinim - the pit was wide, so Yosef would not necessarily have fallen on the snakes, and they would not necessarily have killed him.
Since that would have caused Yosef more suffering than he was destined to suffer. Refer to 37:25:2:2. (EC)
Even if the pit was too narrow to swim properly, Yosef could tread water! Granted, one can do so only for a limited time. Why is this worse than leaving him to starve, which is considered only Gerama? This requires investigation. (PF)