Hi
We went on a Nile Cruise last year and all the coaches used had toilets on them, the coaches did have plenty of stop offs were toilets are available, most transfers from boat to sites where quite short.

Coach transfer to Abu Simbel as an extra excursion was the longest, people did use the coach toilet but it got a bit smelly

all tour coaches have toilets but do yourself a fave if you have to pay extra for excersions shop around you will get the same excersion at half the price instead of throught the rep