I don't think they'd let you do that. When you get your cruise documents, it says specifically that guests are not allowed on board, due to security considerations. They are even more strict about this given recent world events.
In any case, this would not really give you an idea of what the ship feels like on the open sea. When it's tied to the pier, it is in a sheltered harbor, and there is really no difference between being on the ship and in any other tall building. Even when the ship first shoves off, if you aren't looking at "dry land," you won't notice the ship is moving right away. However, the ships are really very stable. Even at sea, you often forget that you are on a ship. They are equipped with stabilizers, which are "wings" which protrude from hull, about 30 feet long, and greatly reduce the ship's motion. I'm told that it reduces it by 90%, but I'm not sure I believe that.
Maybe a 3-day cruise might be an option for you. You could feel out how you like cruising, and if it doesn't agree with you, you won't be "captive" on the ship for a week. On the three-day cruises, the ship is in port all day for both days, so the actual travel on the ship is somewhat limited.