The beach underneath the LAX takeoff route is Play Del Rey. You'd be amazed at how low those large airliners look when taking off right over your head.
The end of the runway runs into dunes that drop off to a road below. Below that is the beach. It's hard to estimate how far the drop is, but it may be as little as 100 feet or so. Add a few hundred feet to that and you've got a 747 taking off three hundred or so feet over your head.
A large bottle rocket, the industrial size (LOL, the kind I've seen in Mexico, probably available in the US in places) might actually be able to reach that high.
Taking off, the pilots are probably in a nose up position that would make it seem like they were a lot farther over water than they actually are. A person on the sand might be able to launch a bottle rocket and cause some concern for pilots.
Gotta tell ya, this is one guy who wouldn't be testing that theory in this day and age.
I wish I knew how to put video on the web. My son and I put a four foot/15 oz model rocket waaaaay over a quarter mile up. Made out of materials and prepackaged engines available at the local hobby shop. The high power rocketry hobbyists routinely put stuff up between 20,000-40'000 ft. Scary