the.com/sea legs

means The physical adjustment that lets you balance and move naturally on a rolling, pitching ship — and the wobbly disorientation that follows when you step back on solid ground.

from A straightforward nautical coinage from sailing's age of wooden ships: new sailors who stumbled and staggered on a heaving deck were said to lack 'sea legs,' while veteran seamen who could walk a tilting deck as easily as a city street had 'found' theirs. The phrase has sailed on into general use for any stretch of adjusting to unfamiliar, unsteady footing — literal or otherwise.