What is incumbent upon me after killing an insect with water to get rid of it, and is that considered a grave sin?

The jurists are of the opinion that it is not forbidden to kill insects, even if they are not harmful, nor is it forbidden to burn them. Rather, it is permissible to kill them if they cause harm.

The school of thought holds that burning ants is disliked (makruh), not forbidden (haram), and the dislike is removed in case of necessity.

Based on this, there is no sin upon you.