The builds I laid out in my guide are designed with kamihime through August 2017 (DMM; 3rd Miracle ticket) in mind. My general philosophy is to plan within a 6-8 month timeframe, relying on the kamihime you'll have sure access to through Miracle tickets.

Water has many ways to build a functional team, which is probably its main strength. Dark can be very strong with the right limited kamihime, and has no elemental weakness, but has limited access to class B debuffs. Offensively Dark has more potential; however, Water gets earlier access to its +100% eidolon, and it has no strings attached. Water has more defensive options and situational counters. Honestly, if you want the ideal Water team, starting with Rudra is probably the way to go. She'll be released in about a month.

The next Miracle ticket is expected in late March / early April. It's not clear what kamihime will be available with it.

There is no alternative for Dark Amaterasu. She fills a niche in Dark that no one else can replace.