Small boats are always at danger of getting run over by larger vessels. Kayaks, being very small and quite low to the water, are particularly vulnerable and difficult to see.

In the summer of 2003 and 2004 a group set about trying to figure out what was the best way of making their small craft show up on radar.

Of the devices tested the winner was ... wait for it ... a tinfoil hat.

The paper is a good read even for people who don't go kayaking in the open ocean. It pretty well outlines the survival basics of how radar works, how radar reflectors work, or fail to work, and what can be done to make any reflector work better. The same basic dynamics are true if your riding a kayak, a 30' sailboat, or have been set adrift in a life raft.

And yes, it also points out that tinfoil hats have more uses that just keeping the NSA satellites from reading your thoughts.

Read all about it at:
http://www.seagrant.umaine.edu/files/pdf-global/05raref.pdf

There is also a more comprehensive study, but far less fun study because they left out the tinfoil hat, of commercial radar reflectors at: (Looks like I blew the address the first time. The correct address is):
http://www.maib.gov.uk/cms_resources.cfm?file=/Radar%20reflectors%20report.pdf

Brought to my attention at:
http://www.cruisersforum.com/forums/f122/radar-reflectors-35612.html

Edited to correct address of second pdf.