Skin is not "hydrated" from the outside.
While there are a very few substances that can penetrate the epidermis and dermal layers (DMSO, Vit E), moisturizers, skin creams and the like are not among them.
Hydration of tissues has to do with water distribution between the extracellular and intracellular compartments, which is regulated by the kidneys in responseto a number of interrelated factors (sodium levels, etc).
Pick up an anatomy text and read a bit about the MULTIPLE layers that skin is comprised of. Read about the tight junctions between cells, tight basement membrane, and keratinized outer layer. Creams and accelerators do not penetrate this layer, and do not stimulate melanocytes. Thus, they do not stimulate tanning. They do grease down the outer layer of dead cells, but this too has no effect on tanning.