Can I use an iPad on the beach safely?
Yes you can, but there are some aspects of the environment of the beach life that need to be controlled.


Salt water corrodes electronics fiercely. Waves crashing means that the atmosphere at a beach is saturated with water droplets with a high concentration of salt in many cases.
Direct sun on the front black face of any iOS device can over heat it rapidly. You will probably feel it uncomfortably hot before the point where the device shuts itself off to prevent battery damage. Even if the device is powered down, you can permanently damage any Lithium Polymer battery in a few hours if the temperature is hot enough.
Sand can get in the home button mechanism and may never come out. The 30 pin connector, the headphone jack, a SIM slot are easier to clean and less likely to be functionally impaired than the close tolerances of the glass on glass home button gap. 
Bright light makes it harder to see the backlit screen and if you are wearing polarized sunglasses, you will see that the display is highly polarized. It's fun from a science experiment aspect, but bad for reading or color accuracy of the display.


I would probably have a Chef Sleeve or two with me and bring an umbrella or shade to control the light and direct heat. A ziplock bag also works if you don't think the closer fitting sleeve is worth spending money on. Depending on your beach some or all of these concerns may be disregarded, but I tried to cover the common beach scenarios.

Other than that - keep it cool, heat yourself up and enjoy your iPad on the beach.