Tubular polystyrene float/bouy for about $6. Length of non-shattering PVC conduit (about 1 or 1 1/2 inch). A counterweight that can be screwed into the bottom of the conduit (which sticks out by at least two feet from the bottom of the float). I used a short length of stainless tube that I sat a long bolt in (so the threads protrude out past the ube), I stood the tup up on its end with the bolt in the middle (on a steel sheet). and then filled the cavity with melted lead.
I inserted the nut for the bolt into the base of the conduit and epoxied it in place. The weight then screws into the nut and stays on the bottom of the conduit.
attach your strob in whatever manner you wish (I like the strobe to actually be in the water, as I find it easier to see the signal when the water acts as a lens), the top of the conduit just has a rescue orange flag on mine.
The entire thing is (before adding the flag) is then painted in a UV resistent brightly coloured
paint.
Job done.
sasha