Sailing.
Technically you hoist a certain letter flag ‘Q’ to declare you are free of disease, and customs officers are invited aboard to check
https://en.m.wikipedia.org/wiki/Pratique
In practice, I have sailed from Harwich, England, to Ostend, Belgium, to Vlissingen,Holland and back on a yacht, without encountering a customs officer anywhere.
We arrived in Belgium on 9/11 so just as the GWOT started
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