Sol and Cthulhu is half an optimal Water team. I'd actually go with that. Pick up SSR Nike with the first Miracle ticket and your Water team will be nearly complete.

Advent events are important for the SSR eidolons, which will be a huge stat buff early on. Save your half-elixirs for your primary element starting out. Only skill level SR+ assault weapons of your primary element, and don't skill level copies you'll use for limit breaks later past LV5.