Paint your boat white with a big red vertical strip near the front? Of course, that might be asking for trouble sometimes...
I would think the passive infared would go off way too often and not deter the real hardened types. Looking like you have nothing worth coming after is my only real answer also. The more you put up defenses, the more evidence you offer that you are protecting something extraordinarily valuable (even if you aren't).
Newport 17 - "Kohanna"
At sea Darwin's hypotheses is the final arbiter of right of way.