Sunscreen should be replaced every year - it can become ineffective.
Is it water-resistant? It should be applied 15-30 minutes prior to exposure and reapplied every 2 hours, and also after perspiring or swimming.
Are you applying enough? The average-sized person should use 1 oz. (about the amount that would fill a shotglass) for their body.
Does it protect against UVA & UVB rays? To protect against UVA, it will contain benzophenones, oxybenzone, sulisobenzone, avobenzone (Parsol 1789), titanium dioxide, or zinc oxide. The best are Parsol 1789, titanium dioxide, or zinc oxide. To protect against UVB, it will contain one or more of the cinnamates (octyl methoxycinnamate and cinoxate) and/or salicylates (homomenthyl, octyl, and triethanolamine).
You want a sunscreen with at least a 30 SPF. Lotions last longer than gels.
Some good water-resistant ones are Bullfrog, Iguana, Ocean Potion Body Wax, & Hawaiian Tropic makes one that is water-resistant, too.
Also, sunscreen will still allow some rays to get through, so you may still tan.
The best you can do is apply the sunscreen & try to stay under an umbrella - especially from 11am-2pm.