You will always move a bit when engine is running and exhaust is flowing. Should be no problem as long as it is not going to far, no more than holding your place against waves.
Your PWC will idle at a couple of mpg at idle. The engines are linked to the pump by a driveshaft 1:1.
The only way mine will idle backwards is when I kick a diverter over the jet drive outlet with a handle on the right side under my throttle/handlebars. Reverse is nice to have.