Most ships provide some bathroom amenities, usually liquid soap (body wash) and shampoo, or sometimes 2-in-1 shampoo/conditioner, however if you are looking for higher end products, you’re not going to find them on the major cruise lines. 

On many ships, there is a dispenser in the shower, filled with body wash and a shampoo/conditoner that some passengers find adequate. 

If a stateroom happens to be out, you can usually ask the room steward and they will get it for you.

Many cruisers choose to bring their own shampoo, conditioner and soap, especially if they have sensitive skin and unlike the airlines, there is no bottle size limit.