A very good topic.
Without going into exhaustive maritime regs, and jurisdiction, of which I am a poor student anyway, I use these guidelines:
stationary (unblinking) white light tells the other vessels there is another vessel there - which is Job #1 as far as this topic.
blinking white is reserved for distress - only. People who use blinking white for regular paddling are inadvertently causing a great deal of confusion. If anything the responding vessel may get very close trying to assist in what they perceive as a distress situation. This might cause a capsize or collision. A blinking white may also set off a chain of Channel 16 emergency calls which ties up Coast Guard and other resources for no purpose (on waters monitored by USCG & other mariners)
if it's attached by a suction cup, it's not attached. Unless a light is lashed to you or your boat don't count on it to come up w. you or stay on the boat in turbulent water.
LED headlamps do limit your vision. Your eyes will adjust tho, and they can be turned to the side or your head, or even the back of the head if there is ambient light (full moon paddling).
They are higher up than deck lights and more easily seen. Many also come w. a flashing option.
This does NOT mean each paddler shouldn't have a small strobe light pinned onto their PFD. That is your ultimate backstop light esp. if you go into the water and get separated from your boat.
Cyalume sticks are nice for group paddles in calm water. Not nearly as bright as LED lights. Wouldn't rely on them to be seen by other vessels.
As others said, kayaks are very low in the water, disappear for intervals in waves, and generally very hard to see by other vessels. Power and sailboaters say they see the paddles moving before anything else so my paddles wear big reflective silver stickers on both sides.
It's also good to invest in a PFD w. plenty of SOLAS tape (or sew some on) as that is very visible from a 1/4 mile off if a light source hits it. SOLAS was designed for SAR personnel to find people in the water.
Night or day, we should follow the Law of Tonnage - he's bigger, get out of the way. Cross channels, etc. as a group and do it in a quick,direct & obvious course. Kayaks are way more manoevrable than a barge or tug, so we should use that advantage and give other vessels the room they require to turn, etc.
A really good topic w. many good suggestions.