Actually, when I was in Nassau (on Monday, May 1, 2006) the Disney Wonder was the only ship docked there that day. I don't know if that is the case every Monday. If you are on a 3 night cruise, you'll be in Nassau on Friday rather than Monday, and there are more ships docked there.
As for walking to the adult beach: the sign said 26 minutes from the family beach. However, there is a tram that takes you to the adult beach, which my girlfriend and I (and it seemed almost everyone else that went to the adult beach) used to get to the adult beach. And we used the last tram of the day to get back (we were among the last people to leave the beach; I guess you can say that she and I really enjoyed it!)
There is also a tram to the family beach. However, that was a nice walk, so my girlfriend and I just walked it, rather than using the tram. Although, at the end of the day, we took the tram (we were also among the last people to leave the family beach!)