I usually go to the tanning spa about a month before the cruise, go once a week until the last week before the cruise. Then I go twice, just to get the basic base. It works great for me, but you still need to be extra careful once in the sun down there.
While I don't recommend a lot of fake'n bake, I do a couple of tanning sessions pre-cruise ONLY to avoid burning when we hit the heat. I will still look pale compared to some, but that is preferrable to looking like a piece of old leather...never mind the skin cancer threat. Even though it doesn't seem really hot your first day or two on the ship, you can burn quickly. Use lots of sun screen.