to patch a hole or repair a leak in an object

(proverb) Literally "to start mending a leak only when the boat is in the middle of the river"; to delay taking action until it is too late to save a situation.

(idiom) to try to fix a problem when it is already too late; to wait until disaster strikes before taking action; literally meaning to wait until a boat is in the middle of the river to fix a leak

It is too late to take precautions once the trouble has already happened; waiting until the last minute to act is useless.