I know that if I gas my V-Ride, it will lurch into reverse very hard. I only move the throttle a touch into reverse off of idle and that is usually all I need. I don't think it's normal for it to feel like it's going no where when you give it a lot of gas in reverse. The boat will often want to spin in circles instead of driving straight backwards, but you should feel it moving regardless. You might want to have the dealer give it a once over just to be sure.
Now, as for driving slow, that is a good thing. You can't maneuver a v-drive like a jetski (as I discovered my first time out), so slow and steady is just how you get around the dock and the launch. Most folks around here just drift out or drift in, no one gets angry because you aren't power sliding right into your slip.