Op I completely sympathize with your concern!
As many of previous posters have mentioned, there are guards, rails etc to prevent that sort of thing. To fall off a ship, you have to be DETERMINED or have help. (I know that sounds a little morbid / nefarious, but 'accidents' don't really happen.) Anyplace that is for passengers will have railing. There are danger zones only accessible to crew. (For example, if you ever had dreams of pulling a Kate/Leo 'flying' scene, don't count on it.)
In cases of bad weather, certain portions of the ship can be off limits or in extreme cases no one is allowed on deck.
I have been on transatlantics (new york to southhampton) where the waves got as high as the ship and rolled over it. (But this was on an ocean liner built for the type of weather.) Everyone was inside and safe when the weather was super bad. What worried me more was my sea sickness.
Most cruises you arent going to encounter crazy wave weather, and if you do have hiccups like this on your cruise, your safety is important to the crew staff too. I was worried about my DS when he was a toddler because of his lack of fear and LOVE of near death adventures. I felt completely OK with him on Disney.
You will be fine! Enjoy your cruise and have a great adventure!