Actually, a more realistic problem is damage caused by running into the things. They're basically logs floating awash. The ones I've seen have a small mast with

Actually, a more realistic problem is damage caused by running into the things. They're basically logs floating awash. The ones I've seen have a small mast with a flag on them, and you could certainly fit them with a radar reflector, but they're still really low in the water and very hard to see. And they're certainly not smart enough to spot an oncoming vessel and take evasive action. Run into one in a fibreglass yacht and you could be having a very bad day.