Another option to consider is rental. Special Needs at Sea offers rentals of both rollators and electric scooters at many US ports. (specialneedsatsea.com) We have rented scooters several times and hubby loves them because he doesn't get tired as easily, has fun joking around while riding and they are conveniently found in your cabin when you check in. (you leave them there when you check out)
When we get to the pier, we always get wheelchair assistance through the pier and its waiting lines. A crew member pushes it through with him in it. All you need to do is ask for the service. If no wheelchair is presently available at the door, then they will ask you to sit in a folding chair until a crew member returns with an empty wheelchair.
The only disadvantage with a scooter is that you need a handicapped room or a large regular suite to accommodate the space to stow them in you cabin. We know that they fit in a HAL Vista suite (on some of their newer ships) because the sofa is placed before the bed (which is next to the balcony window).
Also note that most cabin doors are 23" unless they are handicap accessible and then they are 32".
Generally, rollators and folding wheelchairs can be taken on a tender if you're capable of doing steps to get to the tender. Electric scooters are too heavy, they will not pick them up. We just found out the Celebrity's newest ships (Edge, etc) have a 'Magic Carpet', a flat platform that takes 100 people directly to the tender for boarding (no steps!). Anyone have any experience with this?