Steve is correct for sure! You don't have to do a dock start. Seems you find the Intermediate, advanced, and outlaw class dock starting and the others deep water start. It's no biggy and nobody is going to make fun of you. Most of the people at these comps are just happy that you came out to have fun with everyone else.
BUT, if you want to dock start......
then you need to make sure you jump off the dock with good timing. Mostly it's the boat driver that can mess things up. Either way, you always let the rope start to tighten, and as it's just about to get at "max line tention", you time it by jumping in the water and KEEPing your knees bent while leaning back a little bit. If you lean back too much then you will sink. However, if you start to sink, just stay seated with your knees bent (leaning back a little bit) and you will pop back out. I see most people jump off the dock with good timing, but they always lean to far forward and face plant or they lean to far back and fall on their arse.
Of course you can just wait Super late and let the boat pull you into a railey right off the dock and get mad points.