In Houston, we used 12" styrofoam balls... painted them with florescent orange paint...cut them in half and drilled a 1/2" hole thru them...
Boom... two bouys. and they floated flat on the water, nice and easy to see.
We pulled a floating nylon rope up thru the center with a clip tied on one end and double knot on top of the bouy. clipped it on to the sub bouy, easy and ready to go.
Most of the time when a boat just clips them, it will roll off to the side and the bouy stays in place. On a direct hit, it will pull the knot thru the bouy... easy to fix.. because the rope is still floating... retrieve boat clips on a new bouy,, brings in the one that broke away and re-ties it and it's ready to go again.
In the event of a center punch, the styrofoam is soft enough to minimize damage and save some sponsons and boom tubes...
We always kept 5-6 of them on hand, ready to go on race day so it was a quick fix. kept things moving between rounds.
At one time we tried one gallon milk jugs filled with expanding foam.... bad idea..the foam gets pretty hard after a while and can damage boats.