Travel insurance is strongly recommended for any international trip, and a Nile cruise is no exception — it typically covers trip cancellation or interruption, emergency medical care abroad (which regular health insurance often doesn't cover outside your home country), and lost, delayed, or damaged baggage.
What to check for a cruise specifically
It's worth checking whether the policy covers cruise-specific scenarios — a missed departure, itinerary changes, or medical evacuation from a location that isn't a major city — since the itinerary does move through less accessible areas between major stops.
Some cruise operators require proof of insurance as a condition of booking, so it's worth arranging this early rather than as an afterthought close to departure, when options and pricing are more limited — see What Vaccinations Do You Need for Egypt? for the related pre-trip health planning.