Alcohol is generally available for purchase on most standard and higher-end Nile cruise ships, typically through the bar or during dinner service, though it's usually charged separately rather than included in the base cruise price — some higher-end packages do include a drinks allowance, so it's worth checking specifically.
What varies by ship
Policies do vary by operator and ship, and some smaller or more traditional vessels, including some dahabiyas (see What's the Difference Between a Nile Cruise and a Dahabiya?), may have a more limited selection or none at all — it isn't a universal guarantee across every ship on the river.
Outside the cruise itself, alcohol is available in licensed hotels, tourist restaurants, and designated shops in major cities, though it's less visible or available in more local, non-tourist settings, consistent with general norms in a majority-Muslim country.