After a brief search on Google, I found the following in the Jewish Encyclopedia, which I quote:
Michael is snow, Gabriel is fire (Lüken, l.c. p. 55; comp. Yoma 21b, bottom). Nevertheless, it is the prince of fire and not the prince of ice that is commissioned to rescue Abraham as well as Hananiah, Mishael, and Azariah from the fiery furnace (Pes. 118a; Ex. R. xviii. and parallel passage). In a single passage only (Targ. Job xxv. 2), Michael is called the prince of fire, and Gabriel the prince of water. As prince of fire Gabriel is also prince of the ripening of fruits (Sanh. 95b). As an angel representing an element of nature he is also connected with the metals: Gabriel is gold (the color of fire), Michael is silver (snow), Uriel is copper (Yalḳ., Ḥadash, s.v. "Gabriel," No.75). Gabriel, girded like a metal-worker, shows Moses how to make the candlestick (Men. 29a). He has wings, like all the angels, but while Michael reaches the earth in one flight, Gabriel requires two (Ber. 4b, bottom).
I realize that this links Michael to snow, not water. The Jewish Heritage Magazine also echoes this idea, quoting Job 25:2 and Deut. Rabbah 5:12 in support. Still, snow is close to water, so that may be how Conway got the idea.