I think the best one is leaving bait in the boat like herring in a cooler...

But my favorite rookie mistakes (I've never done any of them just FYI) are:

1.Not putting the plug in the boat before you launch it, then you launch it, and then you realize you are just screwed and you rush it back to the launch to get it back on a trailer....

2. Forgetting Boat keys: self explanatory

3. Picking up the downrigger ball and going to put it on the clip when its hanging over the boat but the downrigger spool is loose as s**t so when you clip it on the ball just goes tearing down.

4. Going in to dock and you have fixed mount downriggers that hang over the boat and you go in and as your docking your downriggers become your bumbers against the pilings.

5. Screwing up a net job...