What does "doesn't have both oars in the water" mean — and why is it funny?

informal, humorous

Meaning

A jokey way of saying that someone is not thinking clearly, or is a bit foolish.

Where it comes from

A modern comic insult from the simple image of rowing a boat — with only one oar in the water, you cannot go straight.

Why it is funny

The humor is the helpless spinning the image promises. You picture someone rowing hard with one oar uselessly in the air, going in slow, earnest circles and getting nowhere.

Used in a sentence

"Ask him to plan anything and it's clear he doesn't have both oars in the water."