Optics! Read here
XKCD: What-If on Hubble for a discussion of "What if we pointed Hubble at the Earth?" Hubble is certainly not designed to look at the Earth. I don't know what today's camera satellites are capable of.
Most 'spy sats' are in orbits chosen for frequent coverage (Low orbits) or station-keeping (High, geosynchronous). Photography from satellites is difficult, but not impossible. Target acquisition, though, is. They'd be looking for small shiny objects in a large, shiny ocean. Most cameras (IR, visible, etc) would only work in daylight. Radar probably doesn't have the range to reach from orbit (Without toasting nearby objects).