Yes - mines and enemy saboteurs diving in naval harbours.
If you believe the conspiracy theorists they are also armed with toxic darts to put the saboteurs to sleep so they would then float to the surface, and a number of them escaped when hurricane Katrina hit the US coast:
http://www.guardian.co.uk/world/2005...sa.theobserver
At least we now know who was on the grassy knoll...