The compartment at the stern of the boat, where the shore power is located-while under way what do you do to keep this hatch from popping up and down during rough conditions?

only put stuff in there that is heavy!!! I never noticed it bouncing up until I watched helicopter video of our boat in a poker run.

On my 419, I tightened the bolts on the hinges and then drilled a small 1/8" diameter hole in the hing on one side and put a pin in it when I know its going to get rough.

That's an interesting question, I must say I have never noticed it to pop up or bounce and I have experineced pretty rough conditions.
How did you come to notice this?

Passenger noticed it and it does not take much to make it pop up.