system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What’s the science behind “sea legs?”
gpt
There are several theories about the specific effect but its not important enough to research properly. On a more general level, the human body likes to maintain a lot of "normal" conditions, because it functions best when it has some standard patterns to follow. For example, jet lag is the body clock's standard pattern being out of sync with the body clocks of the people in your new location and it takes a long time to adjust it because your own body clock is trying to reach your internalised "normal" pattern. 

Our balance and a few other motion senses are regulated in a similar way. The orientation of our head is measured using liquid components in our ears for example and this is used to know whether we're vertical or laying down. It would be annoying for the brain if it was constantly taking in and processing new information about your position due to the bobbing of a boat, so after a while it adds some compensatory information which makes it feel like you're standing still even though you're bobbing. It compensates by adding reverse bobbing sensations. So when you step on land and the bobbing sensations disappear, the reverse bobbing sensations are still there because your brain takes some time to adjust to being on stationary land again and during this time it feels like you're bobbing still.