Even with a tan you are still susceptible to sun burn-tan does not help with burning (done extensive research on sun/sun damage for a project in nursing school) contrary to popular belief...
Wal-Mart carries a brand of sunscreen, called Ocean Potion, that is FABULOUS. I won't use anything else, and it's the only kind I can use on my face that doesn't make it feel like it's on fire... I love whoever made it! They make a sport version too. And it's a big bottle for like $4.50.
*Carnival: Victory 5/13~5/20/2012; Glory 3/13~3/20/11, Freedom 7/26~8/2 09; Destiny 10/30~11/3 '08, 3/2~3/9 '08; Victory 4/15~4/22 '07; Valor 3/5~3/12 '06; Holiday 7/31-8/4 '04; Imagination 6/24~6/29 '02 *Celebrity Equinox 5/14~5/24 '10 *RCI Brilliance OT Seas 6/10~6/22 '08 *Regal Empress 6/16~6/18 '05