Found this on Marella’s faq pages.
14) Can I hire a wheelchair onboard a Marella Cruise?
We don't have wheelchairs available to hire onboard. If you need a manual wheelchair or a mobility scooter, please bring this with you. You can use both manual and electric wheelchairs onboard our cruises, but we do have a limit on the number we can carry for safety reasons.
If you're planning to book a Marella Cruise and want to bring a wheelchair, please call our Assisted Travel Team on 0203 451 2880, before you book, to make sure we have availability. If you've already booked your cruise, you'll still need to call us, so we can check for you. The team will also be able to highlight any restrictions on using your wheelchair onboard the ship.
Due to health and safety regulations, our crew are restricted in the maximum weight they're allowed to lift. This means we're unable to accept any wheelchairs or scooters that don't break down into pieces, with a maximum weight of 20kg per individual piece.
You (or your travelling companion) are responsible for assembling and disassembling the wheelchair/ mobility device.
Calls from landlines cost the standard rate, but calls from mobiles may be higher. Check with your network provider.