I have the same problem. My phone is less than a week old, and I suddenly noticed yesterday that my camera glass has a circular hole in it. The phone has always been in a protective case, and even so, I haven't dropped it anyway.
Just to add another case to this, my s9+ glass also broke right over the 2x lens. Never dropped. Had it since early June.
Makes me think it might be related to temp changes or flexing of the phone over time. Pic is after I removed all the glass, some glue still left.
I replaced the glass by removing all the shattered stuff and cleaning the glue up with alcohol. Replacement glass is under £5 on ebay.
To get it repaired by samsung is £76. Rip off.
Do you know if the phone is still water proof after replacing the lens? I know some take the S9 phone scuba diving and I personally have taken it into the pool. Have you tested after replacing the lens?
I wouldn't take it in chlorinated water or salted water as this can corrode the charging port a waterproof case would be the best bet as for the phone being waterproof after repair I would imagine if Samsung fixes it then maybe but my glass broke twice and fixed it my self for less than £5 and definitely wouldn't take it in water.