I’ve seen small radar reflectors for sale. Should I carry one when kayaking? What else can I do such that boats can use their radar to be aware of my location when they can’t see me due to poor visibility?
It looks like radar reflectors will not hurt but they aren't great either. The bigger and higher they are the better, which is easier to do on a sailboat than a sea kayak. According to one study, a homemade reflector worn on the paddler's hat provided the best signal reflections.
The distance they can be seen regardless of a radar reflector is limited,
At both 1/8 and ¼ nautical mile from the radar platform, kayaks consistently showed up on radar, regardless of whether there was a reflector in use or not. Beginning at ½ nautical mile, kayaks produced an obvious radar signal less than 10% of the time
Bigger kayaks such as tandems and padding a group of kayakers produces a better signal than a reflector.
The tandem sea kayak turned up much better than the solo sea kayaks, regardless of radar reflector.
Kayaks paddling closely together in a pod formation produce a much more significant radar return than a kayak paddling singly with a radar reflector.
The reccomendations were
Based on our field tests, we found that some form of radar reflector, be it commercially manufactured or homemade, is better than none and bigger radar reflectors produce better returns.
Other suggestions for increasing visibility include,
Increase your visibility by wearing bright clothing, using a bright (not dark) colored kayak, paddles with white rather than black blades, and putting reflector tape on your kayak paddles and life jackets.
If you are traveling in a group & you see a vessel approaching, move into a tight group to increase your visibility. Wave your paddles high above your head to alert the vessel operator of your presence.
When crossing a passageway or open water, cross in a tight group and consider using a small radar reflector.
So while a reflector will not hurt, it's not a magic solution either.
Allow me to turn your question on its head a bit. Making yourself more visible to large vessels is a good idea, not being where they need you to be visible is a great idea.
Stay away from channels, shipping lanes, and docking areas. You are small and in a craft that can turn and stop in mere inches- a laden freighter on the other hand, might measure those same parameters in tenths of a mile when in harbor, full miles when in open ocean. Per navigation rules section 27 and 28, such a vessel would be considered either Restricted in Ability to Maneuver or Constrained by Draft, and thus have priority over a kayak regardless of meeting, crossing, or overtaking. The fact that he sees you easily on radar is immaterial. Given that any collision between him and you will go way worse for you, avoidance is the only smart strategy.
Even when dealing with more moderate sized pleasure vessels, keeping yourself aware and leaving them room is still going to more helpful to you in the long run than ensuring they see you earlier. When in situations with reduced visibility, use your sense of sight, sound, and possibly even smell (i.e exhaust) to detect the presence of larger vessels, and turn away immediately.