Don't think that lathering yourself in the morning is enough to last you all day. This isn't just an issue when you're hitting the beach, either. Neglecting to properly moisturize will leave you with dry, irritated skin.
The Fix: If you're in direct sunlight, reapply an SPF of 15 or higher every few hours (at least!) - especially when you're just getting out of the ocean or pool. If you're just enjoying a day about the town, don't forget to moisturize mid-day or at night to keep your skin happy.