Egyptian cuisine has a lot of naturally vegetarian and vegan options built in — dishes like ful medames, koshari, falafel, and a wide range of vegetable-based mezze are everyday staples rather than substitutions, which makes plant-based eating genuinely easier here than in some other destinations.
How this works on a cruise specifically
For Nile cruises, the buffet format helps, since it's simple to pick around dishes rather than needing every meal custom-prepared — see What Food Is Served on a Nile Cruise? for the general spread. It's still worth notifying your cruise operator of any dietary restriction in advance so the kitchen can plan for it rather than improvise.
For more specific needs — allergies, religious dietary requirements, or medical conditions — clear advance communication with your operator matters most, since kitchens generally accommodate what they know about ahead of time far more reliably than requests made on the day.