The Rama in Hilchos Pesach (452:5) quotes the machlokes Ramban and Rashba whether hag'ola can be done using other liquids or whether it must be done specifically with water. The Rama writes that you cannot do hag'ola with any liquid other than water, but bidieved it works even if done with other liquids. The Gr"a points out that our gemara seems to contradict the Ramban because when the gemara tries to figure out the distinction between hag'ola and merika u'shetifa, Rava suggests that the difference is whether one must use water. By merika u'shetifa the Torah explicitly writes ואם בכלי נחושת בשלה ומורק ושטף במים (vayikra 6:26), insisting on water. But for hag'ola one can even use wine. Clearly we see that other liquids are acceptable for hag'ola? The Gr"a says that according to the Ramban we must say that only wine would be acceptable for hag'ola but no other liquid. Or perhaps we pasken like the other distinctions that the gemara makes between hag'ola and merika u'shetifa, not like the Rava. Although the Rama says that bidieved we can rely on kashering using other liquids, R. Akiva Eiger (Teshuvos 83) explains that once food has been cooked in a pot that has been kashered using another liquid the food can be eaten, however one cannot lichatchila cook in a pot that was kashered using another liquid. However, in situations where there is no possibility of kashering with water we can use other liquids since sha'as had'chak is like a bidieved. R. Moshe (Igros Moshe Y.D. 2:41) proves that the Rashba is correct and that other liquids can be used from the gemara in avoda zara 76a (which is also quoted in the gemara zevachim 97a) where each day of cooking a korban shelamim served as a hag'ola for the previous days korban that was cooked in that pot and prevents the absorbed food from becoming no'sar. The cooking of korban meat qualifies as "other liquids", not water, yet it seems to be an acceptable method of hag'ola. One example when it is necessary to kasher using other liquids is in a chocolate factory. Chocolate is very sensitive to water and can be ruined by even a small amount of moisture. Therefore, the only way to kasher from non-kosher (or non cholov yisroel) chocolate would be to do a run of kosher chocolate on the machine, and consider the first run to be non-kosher since it is used as the kashering run.
Raba Bar Ula says that the distinction between hag'ola and merika u'shetifa is that for merika u'shetifa the vessel must be rinsed in cold water after the hag'ola. Tosafos points out that when one kashers using hag'ola it isn't necessary to rinse the utensil in cold water after kashering, but concludes that the minhag is to do so. This is also recorded in Shulchan Aruch (O.C. 452:7) that the custom is to rinse the vessels in cold water immediately after hag'ola. The rationale for this custom is to explain the gemara that merika u'shetifa requires an extra rinsing, implying that even hag'ola requires one rinsing (Tosafos in Avoda Zara 76 quoted by gr"a). Those who don't require a rinsing for hag'ola (Rashi and Tosafos) understand that the "extra rinsing" refers to the cold rinsing that is done after the hot one, and doesn't refer to an extra cold rinsing.