I think the lifeguards may be right, if re bacteira – like E. coli – concentrations being low. I’ve seen lots of plastic bags etc etc etc in relatively clear water; seen murkier water with little refuse: the murkier stuff may be more of a health issue.
I haven’t seen so much glass on beaches; this will depend on location, as likely local origin, such as idiots who drink and then dump n smaks their bottles: polystyrene and so on can come from some distance away.
Very, very hard to get more done about this: I’ve been in HK over 20 years, and throughout there has been some info on reducing rubbish etc, yet pollution issue remains. Some beaches better now: like southern HK Island, thanks to sewage scheme. But others, esp north of west Kowloon, remain grim – inc for bacteria.
I invariably swim w head out of water; try n swiftly spit out any sea water that does get in my mouth.
Also, though beaches and water unsightly and can indeed be filthy, the main health issue is all around us: in the dangerously dirty air.