For a Water team, you'll want at least one healer (eventually). Aphrodite pairs with Ryu-Oh. SSR Nike generally pairs with Cthulhu. Either of your Water accounts will work well with the right Miracle ticket pick.
Athirat allows Shingen to Burst 1T after Provisional Forest. This is useful because it makes it easier to time Bursts to avoid Raging Overdrives. However, it's on a 8T cooldown, so it might not be ready for every Burst. Also, Provisional Forest buffed to double damage by Shingen's Relic weapon doesn't appear to benefit from Athirat's cumulative Burst buff. Water just won't be able to Burst as often as Wind, no matter how you dice it. But in exchange, it can heavily delay overdrives using Charm, Overdrive extension, and 2 Overdrive reductions.
Soul needed is dependent on exactly what team members you have. That'll change depending on what you start with, as well as over time. Mordred is usually a good first choice.
For Dark, most players prefer to start with Satan and another core Dark SSR (e.g., Hades or Osiris). Satan is weak right now, but her Awakening makes her stronger than SSR Amon.