Radar does not work well on objects close to water. And it's easy enough to design a drone boat with low radar reflective shapes(like a rubber dingy with no sharp angles) that would make it even harder. Marine boats are required to have a radar reflector with 90 degree angles so they have a better chance of being spotted, so even your typical sail boat doesn't always show up well on its own.
That said, it wouldn't surprise me at all to know the US military has other technology that could see them.
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