I am not sure if this link will work, but what an interesting bit of kit the Fremantle Sea Rescue is trialling on behalf of Murdoch University.
www.facebook.com/fremantlesearescue/?tn-str=k*F
If it doesn't work..... It is an advanced RADAR that amongst other things has a 100% success rate of finding a person within 1nm in the water in a sea state of 4-5 metres.