Nowadays ships are built using modern technology. A major part of building ship is making it waterproof. So how exactly did ancient ship builders achieve this with access to very limited technology?
Caulking such as horse hair, clay or cloth was poked between the wooden planks to fill gaps. Pitch or tar was applied to the outside to help waterproofing.
Also boats then, like now, aren’t water tight. The bilges would always collect some water. Bailers would use buckets to bail out the bilge water. Today this is done with pumps or stern hatches on some small dinghies/yachts.
Tree resin is an excellent waterproofing agent, and very helpful when binding wood together to create a boat. Sometimes they would use pitch and tar (a resin composite) instead, but resin was the preferred method for many cultures.
Tar/pitch was also used; basically painting/caulking the hull until it kept out most of the water (though some would still get in 9/10 times). Also the whole ‘swabbing the deck’ thing wasn’t a big deal because it was dirty; it was a big deal because wet wood swells to a larger size than dry wood, and larger planks means less space between them, which means it’s less likely to let water in, should it get splashed suddenly
There were and are two main techniques of planking.
Carvel is where you fit the two butted sides together with a good fit and as slight a distance between planks as you can and they lay flush upon the frames and present flush to the outside world. Then, depending usually on the size of the vessel, either cotton or oakum is driven in the whole length of that seam very tightly packed. Once the boat hits water, the planks will swell and compress the cotton which is already in a narrow channel, and the seal will be waterproof. This may take a few hours to days depending on type and state/age of wood and cotton and also length of time out of the water.
Lapstrake or clinker is what you see in Viking ships. The plank above lays over the edge of the plank below. They are both steam bent then carved to receive each other before being fastened together on the boat. They are fastened with rivets which, in the act of riveting, create further compression of the wood along the very accurate fit. A watertight seal.
Both kinds have a rabbet carved into the keel and stem and sternpost (whole spine of the boat) with a complex bevel to receive the garboard (first plank), and cotton caulking along the seam of that rabbet exposed to the world.
Pitch is often poured into the keel at the lowest point around the sternpost so that water which will inevitably get in, from seeping or over the gunwales, will find its way there by gravity but not rot the wood as is sits waiting to be bailed. Pitch can be applied many places and often is for degrees of repairs.
Lapstrake predates caravel so it’s a more ancient technique. I find it simpler in construction too, but that’s just a builder’s preference.
There are more measures that are less intuitive, such as remembering to install a stopwater, which is nothing more than a little dowel of a compatible wood that is driven in cross grain wise at the low point of that spine structure where you will have no access after planking – more or less around where the keel, hog, stern post, and stern knee meet. The point being that if water winds it’s way behind the garboard rabbet deep in the structural joinery, it will hit the stop water which will expand perpendicular to the other pieces -as you put it in against the grain- and it will swell and prevent further channels from starting early stages of erosion and rot.
If we go back even more ancient to the Phoenicians, they started with reed boats made from bundles of woody grasses. I am no expert in their construction techniques, but I think their solution to keep water out of the hold was to layer the bundles thick enough that some would be saturated but the buoyancy physics would mean that at a certain thickness, the high part of the hull would be left dry until loaded with extreme weight. For more information there, I’d suggest reading about Thor Heyerdahl’s KON TIKI where he tried to recreate one with the help of a boat builder from Chad in the fifties then tried to sail it across the Atlantic. I haven’t read it yet but I’m aware of the adventure.