You can expose for the model and let the background blow out, but when the background is very bright compared to the model, you'll probably get a washed out look on the model.. Just set your camera to manual model and meter the model. If you don't have a meter, set your camera on "auto" and zoom in until the model fills your entire screen. Look at the reading on your camera, then switch to manual and set your aperture and shutter to that setting. Widen out and take your shot. Chimp, then adjust the shutter accordingly.
Your on camera flash is good for about 10 feet (in optimal settings). When the sun is that bright ... and you stand 30 feet back ... your on camera flash will be hopelessly overpowered.
What could you have done in this situation? Run to your car and grab your reflective car sunshade (almost everyone in Texas has one in their car) and use that as a reflector. -- OR -- Use a split ND filter to lessen the brightness of the sun on the top portion of your picture. -- OR -- put a polarizer on your lens to cut down on the brightness of the light reflecting off the water -- OR -- Move the model to a shaded area. -- OR -- Wait until the sun drops behind a cloud or until the sun sets more -- OR -- If you can drive your car on the beach, point to into the sun and open a door. Have the model stand so that the reflection from your door adds some light to her face. I like to use the door because you can adjust it easier than other car parts. And, after the sun sets you can use your car's lights to light the model.
I'm sure thqere are a lot of other ideas ... but the key is -- BE RESOURCEFUL
Picture of Jamie below was taken with a 70-200 at f2.8 using my on-camera flash. No meter -- just stood about 12 feet from the model, set the flash on full power, chimped, adjusted the shutter speed, then fired away.