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You can buy the items you mention, but there may only be only one brand of each item available - probably American. When we were onboard in March it was Banana Boat sun cream. Shower gel/shampoo is already in your bathroom in a dispenser on the wall. Basic, but serviceable.
Yes you can small selection in small size.
Yes you can.
Why don't you go prepaed and take all these things with you, so much easie.
We have cruised with norwegien 5 times and you can get all of the items on board plus many others.
Toiletries yes but i do not think you will be able to purchase immodium. i have sailed a couple of times with ncl and found tiletries no problem. i hope you do not need immodiu.