I've also noticed that some of the smaller ones, in this I mean under a metre or so, are not a problem. But whenever they get to about the same size or bigger than me, they become increasingly interested and tend to hang around on the edge of viz and pop in to have a look and I really start freaking out. The shark shield you are talking about, is it a personal version of the shark pod (electrical field device) used by surfers? Do you find it effective and where can one get such a thing? Oh and the inevitable student question, how much of my soul must i sell to get it?
