It is not necessary that you wait.
Showering right after tanning has no detrimental effect on your tan (tingle and bronzers aside). The issue of drying of the skin by showering is so small that it will not interrupt the tanning process, or cause any uneveness in the tan. Melanogenesis occurs from the inside out, not the other way around. And the skin does not "absorb" UV rays, by the way- melanocyte cells are irradiated and begin to produce melanin from tyrosine, this results in delayed pigment darkening (DPD). The first part of a tan is immediate pigment darkening (IPD), which is the photo-oxidation of melanin granules already present. This occurs quite independently from lotions, showers, etc.
Remember, the body has been "tanning" for millions of years, it knows exactly what to do, with or without external factors (lotion companies and salons would like you to think otherwise). So you can shower, not shower, use lotions or not, and you WILL tan. Now about moisturizing, sure it's excellent for your skin, and you can apply it before and after you tan (and shower), and your skin will look and feel great. But in terms of the tanning process itself, if you are properly hydrated (as in drinking plenty H20) your skin will tan properly.