The Koi fish armors block ability seems a bit redundant, Acorn armor provides a similar bonus.  The Koi helmet is almost identical in design to the bubble helmet and if the bubble helmet is able to hold onto a bubble due to surface tension why then can’t the Koi fish helmet.  I just think it would be much better utilized as an aquatic set that gives you water breathing and swim speed plus armor.  Making you a pond samurai.  Just my thoughts though.

To better display the swim bonus I would add two fins to the shinguards just like is present on the gauntlets.

p.s.  It would also be a clever tweak to make the bubble on the bubble helmet/koi helmet (if implemented) to only appear when submerging in water.  I know realism isn’t top of the agenda on a game like this but it could be a cool addition for players to notice themselves.