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I don't know if this helps, you can actually buy lightweight wheelchairs on the internet for £65-£70 inc delivery (google 'wheelchairs'). May be an option.
Check with your cruise line but the ones we have travelled with do have a limited number of wheelchairs available and if this is the case book it in advance and if the tours you book via the cruise line are good they should have reserved seating on the bus. If you do your own thing then if using a taxi it will be better in some countries than others. Another option to make travel easier is to book assist either via yourself and the airline or the tour operator who you are booked this way making it considerably easier boarding/going though security and passport control both going and coming.