oh dear, that might be difficult. i don't want to rain on your parade, but there are a number of technical difficulties with proposing right at the point where ariel and eric get married and getting photos, not to mention photos of you during the ride itself.
first, little mermaid is considered a dark ride, so no flash photography is permitted.
there are segments along the ride in which it is nearly impossible for one clamshell to see into another. it's not as much of a problem at the end when ariel and eric get married.
won't your girlfriend notice if some creepy person is shooting pictures of you throughout the ride?
that being said, if you can accept photos of just the proposal itself when ariel and eric get married, and that there will be no flash so the pictures might be dark, it could be done. you would need castmember help and get a photopass photographer who would ride the clamshell either ahead of you or behind you. you will need to be able to set this up before you and your girlfriend arrive for the ride. i suggest, due to the popularity of the ride, that you do it very early in the day, like first thing in the morning, or when the park is about to close, for that is when the line is shortest and you're most likely to get cast members who can help you.
also keep in mind that at the end of the ride, the clamshells are side by side, so the photographer can only get your picture from the side, which means one of you will have your back to the photographer and potentially would be obstructing the view of the other person.
would you be able to do a test run on the ride to see how you might overcome these technical difficulties? i'm not saying it can't be done, and it is a very creative idea, but if pictures of this special moment are important to you, you'll want to know with some reasonable assurance that the pics are going to turn out okay. you only get to propose to her once.
good luck.