[kianduatha]

Water is so you can cast Streams from Hades and Summon Naiad, which get you into Death and Nature respectively.

I think the reasoning behind Drain-2 is partially to try and mitigate your MR problem against R'lyeh. My preference is towards Magic-1, too, if just because I like Coral Priests as my main researchers(mmm, 45 gold/sacred)