Unless you want to spend half your vacation time ironing, you should unpack everything immediately. Hang shirts and pants two or three to a hanger if you have to (you can always request more hangers later). Just get everything out of your bag before it has a chance to crease.
Pay special attention to the way you unpack and stow your toiletries. If they are not in a waterproof shaving kit or sealed zip-close bag, make sure that you place them in your bathroom vanity and drawers in a way that will keep them stable while the ship is under way. The last thing you want to do is perch a glass bottle of perfume on the edge of a shelf, only to see it tumble over in a seaway, crash down upon the floor, and stink up your entire cabin. Remember, ships move. Your things will, too.
If your cabin has a personal safe, place all your jewelry—as well as your passport—inside of it. Your expensive camera gear, portable DVD player, personal CD player, and wallet should go into the safe, as well. Cruise ships are secure places with professional cabin crews, but you don't want to tempt anyone unnecessarily by leaving your valuables out in plain sight.