Nice game, the focus concept is interesting but needs to be worked on. There should probably have been some sort of visual indicator for the player to see (maybe water droplet dropping into the water as a translucent background?). The water droplet is also quite quiet, you can barely hear it.
If you wanted to make it into more of a reaction based mechanic for the focus, you should have made the focus system a bit more lenient for the player.
But that's just an opinion, overall quite fun and interesting. I enjoyed killing enemies and using the focus mechanic.