There are, apparently, 903 levels of death, and the term vayigva is one of them. According to some, it refers to the simple act of expiration -- the creation of a corpse. Others, like the Rashi referred to (but not cited) here, say it is a term for the death of a righteous person, though this introduces some difficulties.
The Ibn Ezra writes (on Bereishit 25:8)
מלת ויגוע צאת הרוח מהגוף רגע אחד בלי עינוי ועיכוב. הלא תראה ויאסוף רגליו אל המטה (ברא' מט, ג), ומיד גוע. והנה כל גויעה מיתה, ואין כל מיתה גויעה. על כן כתוב במבול ויגוע (ברא' ז, כא)
On the word "vayigva": the exiting of breath from the body in a single moment with no suffering or delay. Compare with "and he brought up his legs on the bed" (49:3) and immediately expired. Behold all expiration is death (mitah) but not every death is [immediate] expiration. Therefore it is written [during the narrative] of the flood "vayigva" (7:21).
The generation of the flood certainly was not righteous -- they were deserving of destruction! Though it might have been a physical truth that they died "quickly" (as posited here) why would it be thought that they merited dying instantaneously and without suffering? Why would the Ibn Ezra write "על כן" 'therefore' when explaining the use of the term in the context of the flood?
In fact, the use of the word vayigva in reference to Yaakov (in the pasuk cited by the Ibn Ezra) in place of "and he died" leads to the medrash that Yaakov actually never died because the soul of the righteous man lives forever! Is this the reward for the generation of the flood?