Accessibility varies a lot both by ship and by site. Some cruise ships have elevators and step-free cabin access; many, especially older or smaller vessels, do not, and boarding sometimes involves gangways or a few steps that can be difficult without assistance.
What the sites themselves are like
The ancient sites present a real mix — some involve stairs, uneven stone ground, or narrow tomb entrances that are difficult to adapt, while parts of Karnak and Philae are more manageable on relatively flat ground with some assistance. See What's the Difference Between Karnak and Luxor Temple? and Why Does Every Aswan Stop Include Philae Temple? for what each site actually involves.
The most reliable approach is to describe your specific needs directly to the tour operator before booking, rather than assuming general accessibility either way — a good operator can tell you honestly which ships and itineraries will actually work.