I'm thinking the water should be random each game to make it different. Maybe the water spaces should be within the middle 4x4 squares

There could be a spot of water on the board where you can fire over, but you can't cross.

Being able to fire two arrows or more in one turn would make it much easier to trap a piece. That might be okay if the piece had some drawbacks too, for instance a rook piece being able to fire 2 arrows in a turn.

What about having the two other gothic chess pieces in the game too, but the knights would not be able to jump. Only if they can make their 1-2 or 2-1 with a clear path they can move that way.