To answer most of these questions,,go to ship
website or brochure,because ea.ship is different re details.Some of the larger ships have different amenities than smaller.So hard to make a general statement.
But,,would pack 3 carry ons for 3 people just for ease.
Most all ships provide hairdryers,have laundry room,,dress casually during day(can wear flip flops),,dress better for nite(no flip flops).Some have Drs.(small ones don t)..never on river ships for instance.
For specific questions,call ships toll free number.Good luck and remember to have fun,if you forget something can buy onboard or in port.