Caulking (or calking) was commonly used in history and prehistory as a way to make seams in wooden ships water tight. The vikings used a method called Clinker (or lapstrake), which was overlapping boards clinched together and then riveted together by brass or iron rivets. The more relevant method (for our caulking discussion) for making ships watertight is called Carvel construction.

In Carvel construction, caulking involved using a scraper and hook to clean out the seam between the planks and then using a caulking mallet and caulking irons to pound and compress caulking into the seam. The caulking used was usually "oakum" which was tarred hemp cordage material.

Here is a really neat video of this process.

After caulking, the seams would be covered with hot pitch. Below the water line the caulked seams would be filled with paint. Above the water line white lead was commonly used. This was known as "paying" the hull.

Caulking did not last forever and hulls would need to have caulking repaired, or be re-caulked at intervals.

None of these wooden shipbuilding techniques was perfect at keeping all water out. Ships took on water especially when being twisted or "worked" by severe weather. Pumps, or bailers in the case of the Vikings, were employed to get unwanted water out of the hull.