Low-tech solution:  Black Polypropylene rope floats on water.  Let
them wrap the stuff around their propeller shafts, pulling a bend in the
rope out of a clothespin type clamp switch that turns on an alarm, and
likely they'll still be there when you get there, trying to figure what
went wrong with their prop.

  I Scuba dive, and in the search and rescue group I was in, one other
member was having problems in the Hood Canal area with some bozo playing
slalom at full speed around her basic open water class students with
their fast boat, "diver down" flags and all.  I suggested this to her &
the perpetrators may now (years later) be out of jail - but I doubt
they'll do THAT again.  She put 4 flags up & just added rope between the
flags...  Might work for you.

  Mark, mwillis@nwlink.com

Peter Grey wrote:
>
> I wonder if anyone can help. I have an application where I wish to
> photograph a boat and people at night (IR camera) but need to be able to
> detect that they are there and in which direction (so I can point a camera
> at them). This is to monitor offshore property. I need to get a range of
> some 100 metres reliably. Hydrophones have been suggested but I am concerned
> about directional capabilities and possible interferences. Does anyone have
> any ideas in this field?
>
> TIA,
>
> Peter Grey
> Neosystems