I regularly run along the sea wall and /or beach, and I have never had a problem with theft from cars, and have never seen evidence of broken windows etc. However a cautious approach would still be sensible, as would be the case everywhere else in the UK.
The beach is safe if you are sensible, walk on ridges and you will be walking on sand. Quick sand is not really an issue unless you are being caught by the tide. The little drainage creeks can be full of silty clay and remain sof and wet, so don't expect to get out of them cleanly unless you take a run and a jump.
The main risk is the incoming tide, its a flat beach, the area has one of the highest tidal ranges in the country, meaning the beach floods very quickly. Dont think you can out run it. You will need to be really quick, I suspect few of us are that quick, especially if you being weighted down with photography equipment.