Penny and I use the Kong Wubba, she goes wacky for it. Haven't had an issue with it sinking but it's not a toy she gets to "chew and paly with" at home (well we use a different wubba for that haha).
I used a kong squeaky ball now and then on the dock too.
which competition are you going to? we're prepping for the summer now, classes start in a few weeks and I am working on getting he to love the bumper (for the speed and height competitions)