Xihuitl Domain Spells:
1st level: touch of undeath*, create and destroy water
3rd level: flowsight**, geyser**
5th level: animate dead, wall of water
7th: control water, death ward
9th: red tide**, danse macabre
* my homebrew
** Walrock Homebrew’s Codex of Waves
Starting at 1st level, your mind taps into the dimensionality of Xihuitl’s rivers, and your powers follow its flow upstream and downstream. Your touch spells gain a range of 30 feet if you and your target are standing in the same body of water. In addition, any time you cast a healing spell on a target that’s partially submerged in water, you restore the maximum number of hit points possible to that target, and you gain a swimming speed equal to your walking speed.
When you choose this domain at 1st level, you gain proficiency with martial weapons.
Channel Divinity: Extract Waters
At 2nd level, you learn how to pull life out of other creatures. As an action, choose a creature within 30 feet with water in its body. That creature must make a Constitution saving throw. On a success, it takes 2d12 necrotic damage. On a failure, it becomes desperately thirsty, and seeks out the nearest drinkable fluid for the next minute.
So We Beat On
Starting at 6th level, all your summoned undead gain a swimming speed and a climbing speed equal to their walking speed. If undead under your control are in a body of water, you can cast healing spells on them and the range of your mental commands includes the range of that body of water.
In addition, you no longer have to concentrate on wall of water.
Beginning at 8th level, the attacks of your undead become dangerously desiccating. As a reaction, after an undead controlled by you hits a target that has water in its body with a melee attack, you can force the target to make a Constitution saving throw or gain one level of exhaustion as your undead sucks moisture out of their body. Creatures composed of water-based liquid have disadvantage on this saving throw.
You can use this ability a number of times equal to your Wisdom modifier per long rest.
Waters of Life
At 17th level, you learn how to share the water that lets you live with others. As an action, you can roll any number of your hit dice and heal a creature you touch by that amount. You can even expend hit dice that you’ve already used during a short rest, but for each already used hit dice you expend, you take 2d8 necrotic damage.
Touch of Undeath
1st level necromancy
Casting Time: 1 action
Duration: Concentration, special
You touch the corpse or skeleton of a Small or Medium humanoid, raising it as a zombie or skeleton under your control for a number of rounds equal to 1 + your spellcasting modifier. On your turns, as long is it’s within 60 feet, you can mentally direct it as a bonus action. If not given any orders, it defends itself against hostile creatures. The undead follows one order until given another.
At the end of this spell, the zombie or skeleton collapses back into a corpse or skeleton unless you use a bonus action to touch it and expend another 1st level spell slot to cast this spell again.
At higher levels: When you cast this spell using a spell slot of 2nd level or higher, the duration of this spell triples.
When you cast this spell at 3rd level, 5th level, 7th level, or 9th level, you can summon an additional undead from a suitable corpse or skeleton within 30 feet.
It's really remarkable how one minor change can transform a subclass from situationally useful to pretty decent. I don't think taking away concentration from wall of water breaks the game too much, and it opens up a lot of fun options for this subclass.
I do have issues with necromancy. I think that necromancy is weak, especially at higher levels, even if you're playing the necromancer subclass, which get buffs to his creatures that make them viable as low level minions. I think these buffs should already be included in necromancy spells to give them greater utility against other powerful spells that compete for slots at 3rd level and 5th level.
To be fair to WotC, balancing summons is a tricky tricky business, and I vastly prefer underpowered summons to overpowered summons. Brandes Stoddard has a nice article which addresses some of my necromancy concerns.
Anyhoo, this subclass probably isn't finished yet. The 8th level feature is a marked break from standard 5e design, and might be overpowered given how good exhaustion is.
I think the most worrisome feature of this subclass is the 6th level ability, specifically healing summons in water. That alone might make for a good 8th level feature. Playtesting will tell.