Lyric discussion by WurmFood 

Cover art for The Water Song lyrics by Mountain Goats, The

"water came springing out the side of the wall" "water came down through the cracks in the ceiling,"

"you're the salesman. I'm the buyer. you're the tractor. I am the tire. I'm the glass. you're the water that fills me"

I don't think the bit about the water is meant to be taken literally. I think it's about someone realizing that they can't avoid how they feel about a certain person anymore. The 'water' (representative of his or her love) is breaking through the walls (which could be a metaphor for the walls around his or her heart) and threatening to drown this person.

My Interpretation

@WurmFood Another thing I didn't mention;

"you're the salesman. I'm the buyer. you're the tractor. I am the tire. I'm the glass. you're the water that fills me"

Person A realized that person B completes him or her. They need person B in their life for things to go smoothly.