system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How were wooden ships made waterproof in ancient times?
gpt
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.