Don't chuck away your empty plastic washing up liquid bottles, make Sharks out of em, like so
Cut out sharks dorsal fins from thin plastic or card, about 8'' tall for adult ones decreasing in size to an inch or so for baby sharks will do.
Stick these fins to the w/u liquid bottles nearer to the tapered neck end with plastic glue. Paint the whole thing black, then paint eyes, snout and gaping snapping mouth full of wicked teeth, let dry.
Now comes the technical bit,''ballasting'' You will need some cement, mud or anything heavy that will set.
Convey your sharks to the sink, bath or bankside. Remove the red squirter nozzle from sharks snout and pour in your ballast of choice keeping your shark fin upwards, try half full at first then add a little at a time, the object being to attain more or less neutral bouyancy, the sharks body barely showing on the waters surface with the fin standing up nice and vertical, firm and erect. When this delicate ballasting is is done and set firm and your satisfied with it, pop the nozzle back on for good. Now the idea is to tow your shark or shark family behind your boat to frighten everybody so do do this get some very light, say 1ib fishing line and lots of it as its best to tow them a long long way behind you, as if they're nothing to do with you and are oblivious that its you towing them along. The fine fishing line should be almost invisible and attach to the snout.
A family of homemade sharks can all be towed together being kept apart by either little opposite counter fins under their tummies or by strips of Balsa wood stuck to em underneath to join them up.
Demonstration. Launch your sharks whilst moving slow ahead and pay out the line, about 200 yards of it. When you've taken up the slack of the tow watch the realistic and beautiful V bow waves from the fins, speed up your boat and watch your sharks body rise slowly up, the gentle upward angle of the towline and the tapered bottle neck causes this.
Now cruise along looking unconcerned and oblivious of the flotilla of sharks that are chasing you while all the gongoozlers on the path scream and shout with horror.
You can even surupticiously pull em in and pretend to feed em. That's it for now folks, well its for when your really really bored really. bizzard.