A purpose-built radar reflector is a good idea. Occasionally known as the "Mongolian Ghost Trap" (can't recall where I read that) the idea is to hoist something that purposefully retro-reflects radar.
Radar won't see the target if the incoming beam isn't directed back where it came from (basic stealth concept). There are devices that look like a bunch of corners made of metal to hoist on your masthead.

THese things are important because there are a lot of merchant vessels out there being steered by "otto"
Otto-pilot. Everyone else is asleep.

Another threat to one's nautical well-being can be mentioned. Submarines.
Submarines can't hear sailboats. (or kayaks) A submarine coming to periscope depth could collide with your silent boat and sink you. Might not even hear you go crunch either.

(I came very, very close to hitting a sailboat one night.)