Q: Where do sick boats go to get healthy?
A: To the dock!
Explanation: A dock is a walk-way that extends from the land into the water. Boats can be tied to the dock so they do not float away.
Doc (sounds like dock) is a word used for doctor, medical doctor.
This joke plays on the word dock/doc because boats go to the dock but they can’t go to the doc.
“Doc” is also used in a phrase by Bugs Bunny: “What’s up doc?”