As as been mentioned, this items feels a bit weak, and a bit clunky.
Ki Point vs Reaction
The Monk only has 2 uses for its Reaction: Deflect Missiles and Slow Fall. It is rather unlikely that a Monk would need to use their Reaction to avoid an attack and in the same round also need their Reaction to again avoid an attack or use one of the above features.
On the other hand, the Monk has 20 Ki Points per Short Rest.
Allowing a player to spend a Ki Point to keep their Reaction feels like a trap, to me, if the only other use of the Reaction is Deflect Missiles or Slow Fall.
Ki Point for +Wis to AC
There are 2 things to compare this possibility to:
- Deflect Missiles: 1 Ki Point and the missile is deflected -- though potentially partially -- no matter how high the Attack Roll of the Attacker.
- Shield Spell: 1 1st-level Spell Slot and you gain +5 to AC.
1 KP for +Wis to AC is worse than either of those:
- It's worse than Deflect Missiles because the attack may still connect, fully, and it doesn't give the opportunity to hit back.
- It's worse than Shield because it only affects the one next attack, and who knows how many there could be, and because +Wis is at most 5 in the first place.
I suspect the latter was the whole intent behind spending another Ki Point to keep the Reaction available to continue dodging the next attacks, however it then becomes extremely expensive.
I would suggest, instead, taking a page from the Shield Spell, and just let the bonus stay on until the start of the next turn.
As a bonus, with a +Wis to AC, Deflect Missiles is much less necessary as a defense, so there's really no incentive whatsoever to try and "keep" the Reaction, which streamlines use.
Proposal
Cloak of Swirling water
This cloak is made of water that lazily swirls around its wearer in the shape of a Samue. Attacks made against the wearer pass through one side of the cloak and out the other as if the wearer was entirely made of water.
As a reaction to an attacker hitting you, you may spend 1 Ki Point to add a bonus to your AC equal to your Wisdom Modifier until the start of your next turn.
Note the subtle differences:
- This is not adding the Wisdom Modifier to the AC, but a bonus equal to the Wisdom Modifier. This avoids having to specify that if the Wisdom Modifier was already added it can be added again, because it's a different bonus to start with.
- Splitting the text between description and mechanical aspect makes it easier to refer to the mechanical aspect in play.
One caveat: it may be that the (now expanded) bonus should cost 2 Ki Points instead of 1. Play-testing should tell.