- **solution 1**

> On this day,the wife shot him, held him under water and then hung him up.

Possible explanation

>! "He" wasn't her husband but another person.

So the husband and his wife are

>! both professional killers

They both love

>! killing

so 

>! maybe he promised to marry her only if she would prove that she was a real killer and kill a person after the marriage.

and

> Then they went out on a honeymoon. Wonderful relationship.

makes sense now.

>! "Oh my dear, lets go now and be together forever. I trust you with my life since you proved to be a real professional killer."

PS: Maybe I added a [tag:lateral-thinking] tag to the question in my mind...

- **solution 2**

> They were married this day.

Maybe...

>! ...the wife (let's call her Alice1) married a woman (Alice2)

so

> On this day,the wife shot him,held him under water and then hung him up.

"him" referring to

>! Alice(1 or 2)'s ex-boyfriend (Bob1 or Bob2, depending on whose boyfriend it was)

So we have 4 possibilities:

- >! Alice1 killed Bob1 because he cheated on her.

- >! Alice2 killed Bob2 because he cheated on her.

- >! Alice1 killed Bob2 because she didn't want Alice2 to fall in love with Bob2 again.

- >! Alice2 killed Bob1 because she didn't want Alice1 to fall in love with Bob1 again.