In general, sunscreen is “water-resistant” if it continues reducing SPF exposure after 40 minutes in the water. To be considered “waterproof,” the bar goes up. The product has to retain its rated SPF factor for 80 minutes in the water. So you could go to the beach or river, hop in three or four times for 15-20 minutes, and still be reasonably sure you won't get scorched.
For starters, I'd recommend Aloe Gator’s SPF 40+ Gel ($12.99 for 4 ounces). It meets Food and Drug Administration (FDA) standards for waterproofness, and, although it's expensive, it also fits easily into a pocket or a backpack.