Fear of Naval mines. So, you're out swimming/scubadiving or whatever and you're tired. Your vision is a little blurry as well is the water. You can't see more than 2 metres ahead of your but you can't slow down the pace because you are running out of air..
Suddenly you see something small in front of you, rapidly growing in size as you draw nearer.. It's not working slowing yourself down and you are within feet of the rusty, old naval mine.