Ships are giant hunks of metal and radio emitters. They light up on SAR satellites[0]. Sentinel-1 gets whole earth coverage and a revisit time of 1-3 days[1] with two active satellites. And that's the public stuff, if you can afford a fleet or even some extra fuel to steer them into interesting orbits you can get faster revisits.
[0] https://x.com/hwtnv/status/2031326840519041114 [1] https://sentiwiki.copernicus.eu/__attachments/1672913/Revisi...