I just returned from the Diamond Princess.
The change machine on Emerald deck seemed to be working but I suggest you make sure you have the proper change before you get the machine loaded.
The dryer runs for only 20 min. (1.00) so you'll need more than a dollar just for drying.
On the way home they had their $20. laundry bag special. But those paper bags they supply are small.
I did laundry several times and didn't find the laundry rooms terribly busy.
Sun Princess (2 times), Mississippi Queen, Dawn Princess, Coral Princess (2 times), Regal Princess, Grand Princess (2 times), Diamond Princess, Maasdam, QM2 (15 times)