Sunscreen is a sticky wicket because like a PP mentioned people automatically assume that if the number is higher, it is better. Well, SPF math is weird. Most sunscreens with an SPF of 15 or higher do an excellent job of protecting against UVB - the burn causing ray (UVA - cancer causing ray) Sun Protection Factor (SPF) is the sunscreen's ability to prevent UVB from damaging the skin. If it takes 20 minutes for your unprotected skin to start turning red, using SPF 15 sunscreen, in theory, prevents reddening 15 times longer about five hours. Like I said - in theory...there's interfering factors like application, type (spray, cream), etc.
SPF 15 filters out approximately 93 percent of all incoming UVB rays. SPF 30 keeps out 97 percent and SPF 50 keeps out 98 percent. So, it isn't double. Using anything higher than 50 won't give you more protection, just cost more money. However, while they may seem like negligible differences, you are light-sensitive, or have a history of skin cancer, those extra percentages could make a difference. No sunscreen is 100% blocking and regardless of strength, should not be expected to stay effective longer than two hours without reapplication. Reddening of the skin is a reaction to UVB rays alone and tells you very little about UVA damage you may be getting. Plenty of damage can be done without the red flag of sunburn being raised. People forget that. They think, I'm not burnt - YAY, no damage.
So, the BEST advice is to make sure your sunscreen is broadspectrum -- says it blocks UVA & UVB and for FL, water-resistant, sports type are less likely to sweat off, but still need reapplied. Don't believe the label that says lasts 12 hours! Bull Hockey!